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❚ C H A P T E R 2

Biological Foundations

Heredity, Prenatal Development, and Birth

❚ C H A P T E R 3

Tools for Exploring the World

Physical Development in Infancy and Early Childhood

❚ C H A P T E R 4

The Emergence of Thought and

Language

Cognitive Development in Infancy and Early Childhood

❚ C H A P T E R 5

Entering the Social World

Socioemotional Development in Infancy and Early Childhood

P A R T I

Prenatal Development, Infancy, and Early Childhood

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2.1 I N T H E B E G I N N I N G : 2 3 PA I R S

O F C H R O M O S O M E S

Mechanisms of Heredity

Genetic Disorders

Heredity, Environment, and Development

❚ REAL PEOPLE: APPLYING HUMAN

DEVELOPMENT:

Ben and Matt Pick Their Niches

2.2 F R O M C O N C E P T I O N TO B I RT H

Period of the Zygote (Weeks 1–2)

❚ CURRENT CONTROVERSIES:

Conception in the 21st Century

Period of the Embryo (Weeks 3–8)

Period of the Fetus (Weeks 9–38)

2.3 I N F L U E N C E S O N P R E N ATA L

D E V E L O P M E N T

General Risk Factors

Teratogens: Drugs, Diseases, and Environmental Hazards

How Teratogens Influence Prenatal Development

Prenatal Diagnosis and Treatment

2.4 L A B O R A N D D E L I V E RY

Stages of Labor

Approaches to Childbirth

Adjusting to Parenthood

Birth Complications

❚ SPOTLIGHT ON RESEARCH:

Impaired Cognitive Functions in Low Birth Weight Babies

Infant Mortality

S U M M A RY

K E Y T E R M S

L E A R N M O R E A B O U T I T

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C H A P T E R 2

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Biological Foundations Heredity, Prenatal Development, and Birth

I f you ask parents to name the most memorable experiences of their lives, many immediately mention the events associated with the birth of their children. From the initial exciting news that

a woman is pregnant through birth 9 months later, the entire experience of pregnancy and birth

evokes awe and wonder.

The period before birth is the foundation for all human development and the focus of this chap-

ter. Pregnancy begins when egg and sperm cells unite and exchange hereditary material. In the first

section, you’ll see how this exchange takes place and, in the process, learn about inherited factors

that affect development. The second section of the chapter traces the events that transform sperm

and egg into a living, breathing human being. You’ll learn about the timetable that governs develop-

ment before birth and, along the way, get answers to common questions about pregnancy. We talk

about some of the problems that can occur during development before birth in the third section of

the chapter. The last section focuses on birth and the newborn baby. You’ll find out how an expectant

mother can prepare for birth and what labor and delivery are like.

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L eslie and Glenn are excited at the thought of starting their own family. At the same time,

they’re nervous because Leslie’s grandfather had sickle-cell disease and died when he was

just 20 years old. Leslie is terrifi ed that her baby may inherit the disease that killed her grandfa-

ther. She and Glenn wish that someone could reassure them that their baby will be okay.

HOW CAN WE REASSURE LESLIE AND GLENN? For starters, we need to know more about sickle-cell disease. Red blood cells carry oxygen and carbon dioxide to and from the body. When a person has sickle-cell disease, the red blood cells are long and curved like a sickle. These stiff , misshapen cells cannot pass through small capillaries, so oxy- gen cannot reach all parts of the body. The trapped sickle cells also block the way of white blood cells that are the body’s natural defense against bacteria. As a result, many people with sickle-cell disease—including Leslie’s grandfather and many other Afri- can Americans, who are more prone to this painful disease than other groups—die from infections before the age of 20.

Sickle-cell disease is inherited and, because Leslie’s grandfather had the disorder, it runs in her family. Will Leslie’s baby inherit the disease? To answer this question, we need to examine the mechanisms of heredity.

| Mechanisms of Heredity

At conception, egg and sperm unite to create a new organism that incorporates some characteristics of each parent. Each egg and sperm cell has 23 chromosomes, thread- like structures in the nucleus that contain genetic material. When a sperm penetrates

L E A R N I N G O B J E C T I V E S

What are chromosomes and genes? How do they carry ❚ hereditary information from one generation to the next?

What are common problems involving chromosomes and ❚ what are their consequences?

How is children’s heredity influenced by the environment ❚ in which they grow up?

2.1 IN THE BEGINNING: 23 PAIRS OF CHROMOSOMES

Red blood cells carry oxygen throughout the

body.

Sickle-shaped blood cells associated with sickle-

cell disease cannot pass through the body’s

smallest blood vessels.

chromosomes

threadlike structures in the nuclei of cells

that contain genetic material

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an egg, their chromosomes combine to produce 23 pairs of chromosomes. The fi rst 22 pairs of chromosomes are called autosomes. The 23rd pair determines the sex of the child, so these are known as the sex chromosomes. When the 23rd pair consists of an X and a Y chromosome, the result is a boy; two X chromosomes produce a girl.

Each chromosome actually consists of one molecule of deoxyribonucleic acid—DNA for short. To understand the structure of DNA, imagine four diff erent colors of beads placed on two strings. The strings complement each other precisely: Wherever a red bead appears on one string, a blue bead appears on the other; wherever a green bead appears on one string, a yellow one appears on the other. DNA is organized this way, except that the four colors of beads are actually four diff erent chemical compounds: adenine, thymine, guanine, and cytosine. The strings, which are made up of phos- phates and sugars, wrap around each other and so create the double helix shown in ❚ Figure 2.1.

The order in which the chemical compound “beads” appear is really a code that causes the cell to create specifi c amino acids, proteins, and enzymes—important bio- logical building blocks. For example, three consecutive thymine “beads” make up the instruction to create the amino acid phenylalanine. Each group of compounds that pro- vides a specifi c set of biochemical instructions is a gene. Thus, genes are the functional

autosomes

fi rst 22 pairs of chromosomes

sex chromosomes

23rd pair of chromosomes; these deter-

mine the sex of the child

deoxyribonucleic acid (DNA)

molecule composed of four nucleotide

bases that is the biochemical basis of

heredity

gene

group of nucleotide bases that provides a

specifi c set of biochemical instructions

Nucleotide bases (A = Adenine, T = Thymine, G = Guanine, C = Cytosine)

Strands of phosphates and sugars

Figure 2.1 ❚ DNA is organized in a double helix, with

strands of phosphates and sugars linked by

nucleotide bases.

Humans have 23 pairs of chromosomes, includ-

ing 22 pairs of autosomes and 1 pair of sex

chromosomes.

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units of heredity because they determine the production of chemical substances that are, ultimately, the basis for all human characteristics and abilities.

Altogether, a person’s 46 chromosomes include roughly 30,000 genes (Pennisi, 2005). Chromosome 1 has the most genes (nearly 3,000) and the Y chromosome has the fewest (just over 200). Most of these genes are the same for all people—fewer than 1% of genes cause diff erences between people (Human Genome Project, 2003). Through biochemical instructions that are coded in DNA, genes regulate the develop- ment of all human characteristics and abilities. The complete set of genes makes up a person’s heredity and is known as the person’s genotype. Genetic instructions, in con- junction with environmental infl uences, produce a phenotype, an individual’s physical, behavioral, and psychological features.

How do genetic instructions produce the misshapen red blood cells of sickle-cell disease? Genes come in diff erent forms that are known as alleles. In the case of red blood cells, for example, two alleles can be present on chromosome 11. One allele has instructions for normal red blood cells; another allele has instructions for sickle- shaped red blood cells. The alleles in the pair of chromosomes are sometimes the same, which is known as being homozygous. The alleles sometimes diff er, which is known as being heterozygous. Leslie’s baby would be homozygous if it had two alleles for normal cells or two alleles for sickle-shaped cells. The baby would be heterozygous if it had one allele of each type.

How does a genotype produce a phenotype? With sickle-cell disease, for example, how do genotypes lead to specifi c kinds of blood cells? The answer is simple if a per- son is homozygous. When both alleles are the same—and therefore have chemical instructions for the same phenotype—that phenotype results. If Leslie’s baby had an allele for normal red blood cells on both of its 11th chromosomes, then the baby would be almost guaranteed to have normal cells. If, instead, the baby had two alleles for sickle-shaped cells, then it would almost certainly suff er from the disease.

When a person is heterozygous, the process is more complex. Often one allele is dominant, which means that its chemical instructions are followed while those of the other, recessive allele are ignored. In sickle-cell disease, the allele for normal cells is dominant and the allele for sickle-shaped cells is recessive. This is good news for Leslie: As long as either she or Glenn contributes the allele for normal red blood cells, their baby will not develop sickle-cell disease.

❚ Figure 2.2 summarizes what we’ve learned about sickle-cell disease: A denotes the allele for normal blood cells, and a denotes the allele for sickle-shaped cells. De- pending on the alleles in Leslie’s egg and in the sperm that fertilizes that egg, three outcomes are possible. Only if the baby inherits two recessive alleles for sickle-shaped cells is it likely to develop sickle-cell disease. But this is unlikely in Glenn’s case: He is positive that no one in his family has had sickle-cell disease, so he almost certainly has the allele for normal blood cells on both of the chromosomes in his 11th pair.

Even though Glenn’s sperm will carry the gene for normal red blood cells, this doesn’t guarantee that their baby will be healthy. Why? Sometimes one allele does not dominate another completely, a situation known as incomplete dominance. In incomplete dominance, the phenotype that results often falls between the phenotype associated with either allele. This is the case for the genes that control red blood cells. Individuals with one dominant and one recessive allele have sickle-cell trait: In most situations they have no problems, but when seriously short of oxygen they suff er a tem- porary, relatively mild form of the disease. Sickle-cell trait is likely to appear when the person exercises vigorously or is at high altitudes (Sullivan, 1987). Leslie and Glenn’s baby would have sickle-cell trait if it inherits a recessive gene from Leslie and a domi- nant gene from Glenn.

The simple genetic mechanism responsible for sickle-cell disease—which involves a single gene pair with one dominant allele and one recessive allele—is also respon- sible for numerous other common traits, as shown in ● Table 2.1. In each of these in- stances, individuals with the recessive phenotype have two recessive alleles, one from each parent. Individuals with the dominant phenotype have at least one dominant allele.

incomplete dominance

situation in which one allele does not

dominate another completely

sickle-cell trait

disorder in which individuals show

signs of mild anemia only when they are

seriously deprived of oxygen; occurs in

individuals who have one dominant allele

for normal blood cells and one recessive

sickle-cell allele

genotype

person’s hereditary makeup

phenotype

physical, behavioral, and psychological

features that result from the interaction

between one’s genes and the environment

alleles

variations of genes

homozygous

when the alleles in a pair of chromosomes

are the same

heterozygous

when the alleles in a pair of chromosomes

diff er from each other

dominant

form of an allele whose chemical instruc-

tions are followed

recessive

allele whose instructions are ignored in

the presence of a dominant allele

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Sickle-cell disease

Normal child

Sickle-cell trait

MotherFather

A A

A a a

Figure 2.2 ❚ In single-gene inheritance, a heterozygous

father and a heterozygous mother can have a

healthy child, a child with sickle-cell trait, or a

child with sickle-cell disease.

● TA B L E 2 . 1

Some Common Phenotypes Associated With Single Pairs of Genes

Dominant Phenotype Recessive Phenotype

Curly hair Straight hair

Normal hair Pattern baldness (men)

Dark hair Blond hair

Thick lips Thin lips

Cheek dimples No dimples

Normal hearing Some types of deafness

Normal vision Nearsightedness

Farsightedness Normal vision

Normal color vision Red–green color blindness

Type A blood Type O blood

Type B blood Type O blood

Rh-positive blood Rh-negative blood

SOURCE: McKusick, 1995.

Most of the traits listed in Table 2.1 are biological and medical phenotypes. These same patterns of inheritance can cause serious disorders, as we’ll see in the next section.

| Genetic Disorders

Some people are aff ected by heredity in a special way: They have genetic disorders that disrupt the usual pattern of development. Genetics can derail development in two ways. First, some disorders are inherited. Sickle-cell disease is one example of

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an inherited disorder. Second, sometimes eggs or sperm do not include the usual 23 chromosomes but have more or fewer chromosomes instead. In the next few pages, we’ll see how inherited disorders and abnormal numbers of chromosomes can alter a person’s development.

Inherited Disorders You know that sickle-cell disease is a disorder that aff ects people who inherit two recessive alleles. Another disorder that involves recessive alleles is phenylketonuria (PKU), a disorder in which babies are born lacking an important liver enzyme. This enzyme converts phenylalanine—a protein found in dairy products, bread, diet soda, and fi sh—into amino acids that are required for normal body functioning. Without this enzyme, phenylalanine accumulates and produces poisons that harm the nervous system, resulting in mental retardation (Diamond et al., 1997; Mange & Mange, 1990).

Most inherited disorders are like sickle-cell disease and PKU in that they are carried by recessive alleles. Relatively few serious disorders are caused by dominant alleles. Why? If the allele for the disorder is dominant, every person with at least one of these alleles would have the disorder. Individuals aff ected with these disorders typically do not live long enough to reproduce, so dominant alleles that produce fatal disorders soon vanish from the species. An exception is Huntington’s disease, a fatal disease characterized by progressive degeneration of the nervous system. Huntington’s disease is caused by a dominant allele found on chromosome 4. Individuals who inherit this disorder develop normally through childhood, adolescence, and young adulthood. Dur- ing middle age, however, nerve cells begin to deteriorate, which produces symptoms such as muscle spasms, depression, and signifi cant changes in personality (Shiwach, 1994). By this age, many adults with Huntington’s disease have already reproduced, creating children who may well later display the disease themselves.

Abnormal Chromosomes Sometimes individuals do not receive the normal complement of 46 chromosomes. If they are born with extra, missing, or damaged chromosomes, development is al- ways disturbed. The best example is Down syndrome. People with Down syndrome have almond-shaped eyes and a fold over the eyelid. Their head, neck, and nose are usually smaller than normal. During the fi rst several months of life, development of babies with Down syndrome seems to be normal. Thereafter, their mental and behav- ioral development begins to lag behind the average child’s. For example, a child with Down syndrome might fi rst sit up without help at about 1 year, walk at 2, and talk at 3, reaching each of these developmental milestones months or even years behind children without Down syndrome. By childhood, most aspects of cognitive and social development are seriously retarded. Rearing a child with Down syndrome presents special challenges. During the preschool years, children with Down syndrome need special programs to prepare them for school. Educational achievements of children with Down syndrome are likely to be limited, and their life expectancy ranges from 25 to 60 years (Yang, Rasmussen, & Friedman, 2002). Nevertheless, as you’ll see in Chapter 6, many individuals with Down syndrome lead full, satisfying lives.

What causes Down syndrome? Individuals with Down syndrome typically have an extra 21st chromosome that is usually provided by the egg (Machatkova et al., 2005). Why the mother provides two 21st chromosomes is unknown. However, the odds that a woman will bear a child with Down syndrome increase markedly as she gets older. For a woman in her late 20s, the risk of giving birth to a baby with Down syndrome is about 1 in 1,000; for a woman in her early 40s, the risk is about 1 in 50. Why? A woman’s eggs have been in her ovaries since her own prenatal development. Eggs may deteriorate over time as part of aging or because an older woman has a longer history of exposure to hazards in the environment, such as X-rays, that may damage her eggs.

An extra autosome (as in Down syndrome), a missing autosome, or a damaged autosome always has far-reaching consequences for development because the auto-

phenylketonuria (PKU)

inherited disorder in which the infant

lacks a liver enzyme

Huntington’s disease

progressive and fatal type of dementia

caused by dominant alleles

Children with Down syndrome typically have up-

ward slanting eyes with a fold over the eyelid, a

flattened facial profile, and a smaller than average

nose and mouth.

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somes contain huge amounts of genetic material. In fact, nearly half of all fertilized eggs abort spontaneously within 2 weeks—primarily because of abnormal autosomes. Thus, most eggs that cannot develop normally are removed naturally (Moore & Per- saud, 1993).

Abnormal sex chromosomes can also disrupt development. ● Table 2.2 lists four of the more frequent disorders associated with atypical numbers of X and Y chromo- somes. Keep in mind that “frequent” is a relative term; although these disorders are more frequent than PKU or Huntington’s disease, most are uncommon. Notice that there are no disorders consisting solely of Y chromosomes. The presence of an X chro- mosome appears to be necessary for life.

Fortunately, most of us receive the correct number of chromosomes and do not in- herit life-threatening illnesses. For most people, heredity reveals its power in creating a unique individual—a person unlike any other.

Now that you understand the basic mechanisms of heredity, we can learn how heredity and environment work together to produce behavioral and psychological development.

| Heredity, Environment, and Development

Many people mistakenly view heredity as a set of phenotypes unfolding automati- cally from the genotypes that are set at conception. Nothing could be further from the truth. Although genotypes are fi xed when the sperm fertilizes the egg, phenotypes are not. Instead, phenotypes depend both on genotypes and on the environment in which individuals develop.

To begin our study of heredity and environment, we need to look fi rst at the meth- ods that developmental scientists use.

Behavioral Genetics: Mechanisms and Methods Behavioral genetics is the branch of genetics that deals with inheritance of behavioral and psychological traits. Behavioral genetics is complex, in part, because behavioral and psychological phenotypes are complex. Traits controlled by single genes are usu- ally “either–or” phenotypes. A person either has dimpled cheeks or not; a person ei- ther has normal color vision or red-green color blindness; a person’s blood either clots normally or it does not. In contrast, most important behavioral and psychological characteristics are not of an “either–or” nature; rather, a range of diff erent outcomes is possible. Take extraversion as an example. Imagine trying to classify 10 people that you know well as either extroverts or introverts. This would be easy for a few ex- tremely outgoing individuals (extroverts) and a few intensely shy persons (introverts).

● TA B L E 2 . 2

Common Disorders Associated With the Sex Chromosomes

Sex Disorder Chromosomes Frequency Characteristics

Klinefelter’s XXY 1 in 500 male births Tall, small testicles, sterile, below- syndrome normal intelligence, passive

XYY complement XYY 1 in 1,000 male births Tall, some cases apparently have below-normal intelligence

Turner’s syndrome X 1 in 2,500–5,000 female Short, limited development of births secondary sex characteristics, problems perceiving spatial relations

XXX syndrome XXX 1 in 500–1,200 female Normal stature but delayed motor births and language development

behavioral genetics

the branch of genetics that studies the

inheritance of behavioral and psychologi-

cal traits

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Most people are neither extroverts nor introverts but “in between.” The result is a distribution of individuals ranging from extreme introversion at one end to extreme extroversion at the other.

Many behavioral and psychological characteristics are distributed in this fashion, including intelligence and many aspects of personality. When phenotypes refl ect the combined activity of many separate genes, the pattern is known as polygenic inheri- tance. Because so many genes are involved in polygenic inheritance, we usually can- not trace the eff ects of each gene. But we can use a hypothetical example to show how many genes work together to produce a behavioral phenotype that spans a continuum. Let’s suppose that four pairs of genes contribute to extroversion, that the allele for extroversion is dominant, and that the total amount of extroversion is simply the sum of the dominant alleles. If we continue to use uppercase letters to represent dominant alleles and lowercase letters to represent the recessive allele, then the four gene pairs would be Aa, Bb, Cc, and Dd.

These four pairs of genes produce 81 diff erent genotypes and 9 distinct pheno- types. For example, a person with the genotype AABBCCDD has 8 alleles for extro- version (the proverbial party animal). A person with the genotype aabbccdd has no alleles for extroversion (the proverbial wallfl ower). All other genotypes involve some combination of dominant and recessive alleles, so these are associated with pheno- types representing intermediate levels of extroversion. In fact, ❚ Figure 2.3 shows that the most common outcome is for people to inherit exactly 4 dominant and 4 recessive alleles, and 19 of the 81 genotypes (e.g., AABbccDd, aaBbcCDd) produce this pattern. A few extreme cases (very outgoing or very shy), when coupled with many intermediate cases, produce the familiar bell-shaped distribution that characterizes many behav- ioral and psychological traits.

Remember, this example is completely hypothetical. Extroversion is not based on the combined infl uence of eight pairs of genes. However, the sample shows how sev- eral genes working together could produce a continuum of phenotypes. Something like our example is probably involved in the inheritance of many human behavioral traits, except that many more pairs of genes are involved. What’s more, the environ- ment also infl uences the phenotype.

T H I N K A B O U T I T

Introversion–extroversion is an example

of a psychological characteristic that

defines a continuum. Think of other

psychological characteristics like this,

in which outcomes are not “either–or”

but are distributed across a range.

polygenic inheritance

when phenotypes are the result of the

combined activity of many separate genes

0 1 2

AABBCCDD

AaBBCCDD

AABbCCDD

AABBCCDd

AABBCcDD

aaBBCCDD

AaBbCCDD

AaBBCcDD

AaBBCCDd

AAbbCCDD

AABbCcDD

AABbCCDd

AABBccDD

AABBCcDd

AABBCCdd

aaBbCCDDaabbCCDDaabbCcDDaabbccDDaabbccDd

aaBBCcDDaaBbCcDDaabbCcDd aabbCCDdaabbCcdd

aaBBCCDdaaBbCCDdaabbCCdd aaBbccDDaaBbccdd

AabbCCDDaaBBccDDaaBbccDd aaBbCcDd

AaBbCcDDaaBBCcDdaaBbCCddaaBbCcdd

AaBbCCDdaaBBCCddaaBBccDdaaBBccdd

AaBBccDDAabbCcDDaaBBCcddAabbccDd

AaBBCcDdAabbCCDdAabbccDDAabbCcdd

AaBBCCddAaBbccDDAabbCcDdAaBbccdd

AAbbCcDDAaBbCcDdAabbCCdd

AAbbCCDdAaBbCCddAaBbccDd

AABbccDDAaBBccDdAaBbCcdd

AABbCcDdAaBBCcddAaBBccdd

AABbCCddAAbbccDDAAbbccDd

AABBccDdAAbbCcDdAAbbCcdd

AABBCcddAAbbCCdd

AABbccDd

AABbCcdd

AABBccdd

AABbccdd

AAbbccdd

Aabbccdd

aabbccdd

3 4 5 6 7 8 Number of dominant alleles for extroversion (phenotype)

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Figure 2.3 ❚ Many behavioral phenotypes represent a con-

tinuum (with many people falling at the mid-

dle of the continuum), an outcome that can

be caused by many genes working together.

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If many behavioral phenotypes involve countless genes, how can we hope to un- ravel the infl uence of heredity? Twins and adopted children provide some important clues to the role of heredity. In twin studies, researchers compare identical and fra- ternal twins. Identical twins are called monozygotic twins because they come from a single fertilized egg that splits in two. Because identical twins come from the same fertilized egg, the same genes control their body structure, height, and facial features, which explains why identical twins look alike. In contrast, fraternal or dizygotic twins come from two separate eggs fertilized by two separate sperm. Genetically, fraternal twins are just like any other siblings—on average, about half their genes are the same. In twin studies, scientists compare identical and fraternal twins to measure the infl u- ence of heredity. When identical twins are more alike than are fraternal twins, this implicates heredity (Phelps, Davis, & Schartz, 1997).

A similar logic is used in adoption studies, in which adopted children are compared with their biological parents and their adoptive parents. The idea here is that biological parents pro- vide their child’s genes whereas adoptive parents provide the child’s environment. Consequently, if a behavior has important genetic roots, then adopted children should behave more like their biological parents than like their adoptive parents.

These and other methods are not foolproof. Perhaps you recognized a potential fl aw in twin studies: Parents and other people may treat monozygotic twins more similarly than they treat dizygotic twins. This would make monozygotic twins more similar than dizygotic twins in their experiences as well as in their genes. Each method of study has its unique pitfalls, but if diff erent methods converge on the same conclusion about the infl uence of heredity then we can be confi dent of that result. Throughout this book, you’ll see many instances where twin studies and adoption studies have pointed to genetic infl uences on human development.

Behavioral geneticists are now moving beyond the traditional methods of twin and adoption studies (Dick & Rose, 2002; Plomin & Crabbe, 2000). Today it is possible to isolate particular segments of DNA in human chromosomes. These segments then serve as markers for identifying specifi c alleles. The procedure is complicated, but the basic approach often begins by identifying people who diff er in the behavioral or psy- chological trait of interest. For example, researchers might identify children who are outgoing and children who are shy. Or they might identify children who read well and children who read poorly. The children rub the inside of their mouths with a cotton swab, which yields cheek cells that contain DNA. The cells are analyzed in a lab, and the DNA markers for the two groups are compared. If the markers diff er consistently, then the alleles near the marker probably contribute to the diff erences between the groups.

Techniques of this sort have the potential to identify the many diff erent genes that contribute to complex behavioral and psychological traits. Of course, these new meth- ods have limits. Some require large samples of people, which can be hard to obtain when studying a rare disorder. Also, some studies require that an investigator have a specifi c idea at the outset about which chromosomes to search for and where. These can be major hurdles. However, when used with traditional methods of behavioral genetics (e.g., adoption studies), the new methods promise a much greater understand- ing of how genes infl uence behavior and development.

Throughout the rest of this book, you’ll encounter many instances that show the interactive infl uences of heredity and environment on human development. In the next few pages, however, we want to mention some general principles of heredity– environment interactions.

Paths From Genes to Behavior How do genes work together—for example, to make some children brighter than oth- ers and some more outgoing than others? That is, how does the information in strands of DNA infl uence a child’s behavioral and psychological development? The specifi c

Identical twins are called monozygotic twins

because they came from a single fertilized egg

that split in two; consequently, they have identi-

cal genes.

monozygotic twins

the result of a single fertilized egg split-

ting to form two new individuals; also

called identical twins

dizygotic twins

the result of two separate eggs fertilized

by two sperm; also called fraternal twins

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A la

m y

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paths from genes to behavior are largely uncharted, but in the next few pages we’ll discover some of their general properties.

1. The behavioral consequences of genetic instructions depend on the envi- ronment in which those instructions are implemented. In other words, a genotype can lead to many different phenotypes depending on the specific environment in which the genotype is “expressed” (Gottesman & Hanson, 2005). Reaction range refers to the fact that the same genotype can produce a range of phenotypes in reaction to the environment where development takes place. For example, imagine two children with the same genotype for “average intelligence.” The children’s phenotypic intelligence would depend on the environments in which they develop. If one child is brought up in an impoverished, unstimulating environment, then his or her phe- notypic intelligence may be below average. In contrast, if the second child is brought up in an enriched environment filled with stimulation, then this child’s phenotypic intelligence may be above average. Thus the same genotype for intelligence can lead to a range of phenotypes, depending on the quality of the rearing environment. Of course, what makes a “good” or “rich” environment is not the same for all facets of behavioral or psycho- logical development. Throughout this book, you will see how specific kinds of environments influence very particular aspects of development (Wachs, 1983).

Because of this general principle, you need to be wary when you read statements like “X percent of a trait is due to heredity.” In fact, behavioral geneticists often use correlations from twin and adoption studies to calculate a heritability coefficient, which estimates the extent to which diff erences between people refl ect heredity. For example, intelligence has a heritability coeffi cient of about .5, which means that about 50% of the diff erences in intelligence between people is due to heredity (Bouchard, 2004).

Why be cautious? One is that many people mistakenly interpret herita- bility coeffi cients to mean that 50% of an individual’s intelligence is due to heredity; this is incorrect because heritability coeffi cients apply to groups of people, not to a single person.

A second reason for caution is that heritability coeffi cients apply only to a specifi c group of people living in a specifi c environment. They cannot be applied to other groups of people living in the same environment or to the same people living elsewhere. For example, a child’s height is certainly infl uenced by heredity, but the value of a heritability coeffi cient depends on the environment. When children grow in an environment that has ample nutrition—allowing all children to grow to their full genetic potential—heri- tability coeffi cients will be large. But when some children receive inadequate nutrition, this aspect of their environment will limit their height and, in the process, reduce the heritability coeffi cient.

Similarly, the heritability coeffi cient for reading disability is larger for parents who are well educated than for parents who are not (Friend, De- Fries, & Olson, 2008). Why? Well-educated parents more often provide the academically stimulating environment that fosters a child’s reading; conse- quently, reading disability in this group usually refl ects heredity. In contrast, less educated parents less often provide the needed stimulation; hence their children’s reading disability refl ects a mixture of genetic and environmental infl uences.

This brings us back to the principle that began this section: “The conse- quences of genetic instructions depend on the environment in which those instructions develop.” Both genes and environments are powerful infl uences on development, but we can understand one only by considering the other, too. This is why it is essential to expand research beyond the middle-class, European American participants who have dominated the samples of scien-

reaction range

a genotype is manifested in reaction to the

environment where development takes

place, so a single genotype can lead to a

range of phenotypes

heritability coeffi cient

a measure (derived from a correlation

coeffi cient) of the extent to which a trait or

characteristic is inherited

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BIOLOGIC AL FOUNDATIONS | 51

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tists studying child development. Only by studying diverse groups of people can we truly understand the many ways in which genes and environments propel children along their developmental journeys.

2. Heredity and environment interact dynamically throughout development. A simple-minded view of heredity and environment is that heredity pro- vides the clay of life and experience does the sculpting. In fact, genes and environments constantly infl uence each other throughout a person’s life (Gottesman & Hanson, 2005; Rutter, 2007). This principle actually has two parts. First, genes are expressed—“turned on”—throughout the life span. For example, genes initiate the onset of menstruation in the early teens and the graying of hair in midlife. Second, the environment can trigger genetic expression: A person’s experiences can help to determine how and when genes are activated (Gottlieb, 2000). For instance, teenage girls begin to menstruate at a younger age if they’ve had a stressful childhood. The exact pathway of infl uence is unknown (though it probably involves the hormones that are triggered by stress and those that initiate ovulation), but this is a clear case where the environment advances the developmental clock (Ellis, 2004).

Returning to the analogy of sculpting clay, a more realistic view is that new clay is constantly being added to the sculpture, leading to resculpting, which causes more clay to be added, and the cycle continues. Hereditary clay and environmental sculpting are continuously interweaving and infl uencing each other.

3. Genes can infl uence the kind of environment to which a person is exposed. In other words, “nature” can help to determine the kind of “nurturing” that a child re- ceives (Scarr, 1992; Scarr & McCartney, 1983). A person’s genotype can lead others to respond in a specifi c way. For example, imagine someone who is bright and outgoing as a result, in part, of her genes. As a child, she may receive plenty of attention and encouragement from teachers. In contrast, someone who is not as bright and is more withdrawn (again, due in part to heredity) may be easily overlooked by teachers. In addition, as children grow and become more independent, they actively seek environments that fi t their genetic makeup. Children who are bright may actively seek peers, adults, and activi- ties that strengthen their intellectual development. Similarly, people who are outgoing may seek the company of other people, particularly extroverts like themselves. This process of deliberately seeking environments that fi t one’s heredity is called niche-picking. Niche-picking is fi rst seen in childhood and becomes more common as children grow older and can control their environments. The Real People feature shows niche- picking in action.

4. Environmental infl uences typically make children within a family diff er- ent. One of the fruits of behavioral genetic research is greater understand- ing of the manner in which environments infl uence people. Traditionally, scientists considered some environments benefi cial for people and others detrimental. This view has been especially strong in regard to family envi- ronments. Some parenting practices are thought to be more eff ective than others, and parents who use these eff ective practices are believed to have children who are, on average, better off than children of parents who don’t use these practices. This view leads to a simple prediction: Children within a family should be similar because they all receive the same type of eff ective (or ineff ective) parenting. However, dozens of behavioral genetic studies

Children who are outgoing often like to be with

other people and deliberately seek them out;

this is an example of niche-picking.

ef f

G re

en be

rg /P

ho to

Ed it

niche-picking

process of deliberately seeking environ-

ments that are compatible with one’s

genetic makeup

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show that, in reality, siblings are not very much alike in their cognitive and social development (Plomin & Spinath, 2004).

Does this mean that family environment is not important? No. These findings point to the importance of nonshared environmental influences, the forces within a family that make children different from one another. Although the family environment is important, it usually affects each child

in a unique way, which makes siblings differ. Each child is likely to have different experiences in daily family life. For example, parents may be more affectionate with one child than another, they may use more physical punishment with one child than another, or they may have higher expectations for school achievement by one child than another. All these contrasting parental influ- ences tend to make siblings different, not alike (Liang & Eley, 2005). Family environments are important, but—as we describe their influence throughout this book—you should remember that families actually create multiple unique environments, one for each person in the family.

Much of what we have said about genes, environment, and development is summarized in ❚ Figure 2.4 (Lytton, 2000). Parents are the source of children’s genes and, at least for young children, the primary source of children’s experiences. Children’s genes also infl uence the experi- ences they have and the impact of those experiences on them. However, to capture the idea of nonshared environ-

Children’s experiences within a family typically

make them different from one another, not more

alike.

Child’s environment

Child’s phenotype

Parents’ genes

Child’s genes

Figure 2.4 ❚ Parents influence their children by providing

genes and by providing experiences; children’s

genes and their environments work together

to shape development.

an dy

F ar

is /C

or bi

s Real People: Applying Human Development Ben and Matt Pick Their Niches

Ben and Matt Kail were

born 25 months apart.

Even as a young baby,

Ben was always a “people person.” He relished

contact with other people and preferred play

that involved others. From the beginning, Matt

was different. He was more withdrawn and was

quite happy to play alone. The first separation

from parents was harder for Ben than for Matt,

because Ben relished parental contact more.

When they entered school, Ben enjoyed increas-

ing the scope of his friendships; Matt liked all the

different activities that were available and barely

noticed the new faces. Though brothers, Ben and

Matt are quite dissimilar in terms of their socia-

bility, a characteristic known to have important

genetic components (Braungart et al., 1992).

As Ben and Matt have grown up (they’re now

adults), they have consistently sought environ-

ments that fit their differing needs for social

stimulation. Ben was involved in team sports and

now enjoys working in the theater. Matt took

art and photography classes and now is happy

when he’s reading, drawing, or working at his

computer. Ben and Matt have chosen very differ-

ent niches, and their choices have been driven in

part by the genes that regulate sociability.

nonshared environmental infl uences

forces within a family that make siblings

diff erent from one another

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Recall answers: (1) autosomes, (2) Polygenic inheritance, (3) Down syndrome, (4) the

fertilized egg is aborted spontaneously, (5) reaction range, (6) different from each other

Test Yourself

RECALL

1. The fi rst 22 pairs of chromosomes are

called .

2. refl ects the combined activity of a

number of distinct genes.

3. Individuals with have an extra 21st

chromosome, usually inherited from the mother.

4. When a fertilized egg has defective autosomes, the usual

result is that .

5. The term refers to the fact that

the same genotype can be associated with many diff erent

phenotypes.

6. Nonshared environmental infl uences tend to make siblings

INTERPRET

Explain how reaction range and niche-picking show the inter-

action between heredity and environment.

APPLY

Leslie and Glenn, the couple concerned that their baby could

have sickle-cell disease, are already charting their baby’s life

course. Leslie, who has always loved to sing, is confi dent that

her baby will be a fantastic musician and imagines a routine

of music lessons, rehearsals, and concerts. Glenn, a pilot, is

just as confi dent that his child will share his love of fl ying; he

is already planning trips the two of them can take together.

What advice might you give to Leslie and Glenn about factors

they are ignoring?

2.2 FROM CONCEPTION TO BIRTH

L E A R N I N G O B J E C T I V E S

What happens to a fertilized egg in the first two weeks after ❚ conception?

When do body structures and internal organs emerge in pre- ❚ natal development?

When do body systems begin to function well enough to ❚ support life?

E un Jung has just learned that she is pregnant with her fi rst child. Like many other parents-

to-be, she and her husband, Kinam, are ecstatic. But they also soon realize how little they

know about “what happens when” during pregnancy. Eun Jung is eager to visit her obstetrician

to learn more about the normal timetable of events during pregnancy.

PRENATAL DEVELOPMENT BEGINS WHEN A SPERM SUCCESSFULLY FERTILIZES AN EGG. The many changes that transform the fertilized egg into a newborn human constitute prenatal development. Prenatal development takes an average of 38 weeks, which are divided into three periods: the period of the zygote, the period of the embryo, and the

prenatal development

the many changes that turn a fertilized

egg into a newborn human

mental infl uences we would need a separate diagram for each child, refl ecting the fact that parents provide unique genes and a unique family environment for each of their off spring.

Most of this book explains the links between nature, nurture, and development. We can fi rst see the interaction of nature and nurture during prenatal development, which we examine in the next section of this chapter.

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period of the fetus.* Each period gets its name from the scientifi c term used to describe the baby-to-be at that point in its prenatal development.

In this section, we’ll trace the major developments of each of these periods. As we do, you’ll learn the answers to the “what happens when” question that intrigues Eun Jung.

| Period of the Zygote (Weeks 1–2)

The teaspoon or so of seminal fl uid produced during a fertile male’s ejaculation contains from 200 to 500 million sperm. Of the sperm released into the vagina, only a few hundred will actually complete the 6- or 7-inch journey to the Fallo- pian tubes. Here, an egg arrives monthly, hours after it is released by an ovary. If an egg is present, many sperm will simultaneously begin to burrow their way through the cluster of nurturing cells that surround the egg. When one sperm fi nally penetrates the cellular wall of the egg, chemical changes occur in the wall immediately, blocking out all other sperm. Then the nuclei of the egg and sperm fuse, and the two independent sets of 23 chromosomes are interchanged. The development of a new human being is under way!

For nearly all of history, sexual intercourse was the only way for egg and sperm to unite and begin the development that results in a human being. This is no longer the only way, as we see in the Current Controversies feature.

Fertilization begins when sperm cells burrow

their way through the outer layers of an egg cell.

In this photo, the tails of the sperm can be seen

clearly but one sperm has burrowed so deeply

that the head is barely visible.

*Perhaps you’ve heard that pregnancy lasts 40 weeks and wonder why we say that prenatal de-

velopment lasts 38 weeks. The reason is that the 40 weeks of pregnancy are measured from the

start of a woman’s last menstrual period, which typically is about 2 weeks before conception.

Current Controversies Conception in the 21st Century

More than 30 years ago,

Louise Brown captured

the world’s attention as

the first test-tube baby—conceived in a petri

dish instead of in her mother’s body. Today, this

reproductive technology is no longer experi-

mental; it is used more than 130,000 times an-

nually by American women and produces more

than 50,000 babies each year (U.S. Department

of Health and Human Services, 2007). Many new

techniques are available to couples who cannot

conceive a child through sexual intercourse. The

best-known technique, in vitro fertilization, in-

volves mixing sperm and egg together in a petri dish

and then placing several fertilized eggs in the moth-

er’s uterus, with the hope that they will become im-

planted in the uterine wall. Other methods include

injecting many sperm directly into the Fallopian

tubes or a single sperm directly into an egg.

The sperm and egg usually come from the

prospective parents, but sometimes they are

provided by donors. Typically, the fertilized eggs

are placed in the uterus of the prospective

mother, but sometimes they are placed in the

uterus of a surrogate mother who carries the

baby to term. This means that a baby could have

as many as five “parents”: the man and woman

who provided the sperm and egg; the surrogate

mother who carried the baby; and the mother

and father who will rear the baby.

New reproductive techniques offer hope for

couples who have long wanted a child, and stud-

ies of the first generation of children conceived

via these techniques indicate that their social

and emotional development is perfectly normal

(MacCallum, Golombok, & Brindsen, 2007; Go-

lombok et al., 2004). But there are difficulties

as well. Only about one third of attempts at in

vitro fertilization succeed. What’s more, when

a woman becomes pregnant, she is more likely

to have twins or triplets because multiple eggs

are transferred to increase the odds that at least

one fertilized egg will implant in the mother’s

uterus. She is also at greater risk for giving birth

to a baby with low birth weight or birth defects.

Finally, the procedure is expensive—the aver-

age cost in the United States of a single cycle of

treatment is between $10,000 and $15,000—

and often is not covered by health insurance

(Katz, Nachtigall, & Showstack, 2002).

These problems emphasize that, although

technology has increased the alternatives for

infertile couples, pregnancy on demand is still

in the realm of science fiction. At the same

time, the new technologies have led to much

controversy because of some complex ethical

issues associated with their use. One concerns

the prospective parents’ right to select particu-

lar egg and sperm cells; another involves who

should be able to use this technology.

Pick your egg and sperm cells from a catalog?

Until recently, prospective parents have known

nothing about egg and sperm donors. Today,

however, they are sometimes able to select egg

and sperm based on physical and psychological

characteristics of the donors, including appear-

ance and race. Some claim that such prospective

parents have a right to be fully informed about

the person who provides the genetic material

en na

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ils so

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Whether by artifi cial means as just described or by natural means, fertilization begins the period of the zygote, the technical term for the fertilized egg. This period ends when the zygote implants itself in the wall of the uterus. During these 2 weeks, the zygote grows rapidly through cell division. ❚ Figure 2.5 traces the egg cell from the time it is released from the ovary until the zygote becomes implanted in the wall of the uterus. The zygote travels down the Fallopian tube toward the uterus. Within hours, the zygote divides for the fi rst time; it then continues to do so every 12 hours. Occasionally, the zygote separates into two clusters that develop into identical twins. Fraternal twins, which are more common, are created when two eggs are released and each is fertilized by a diff erent sperm cell.

After about 4 days, the zygote includes about 100 cells and resembles a hollow ball. The inner part of the ball is destined to become the baby. The outer layer of cells will form a number of structures that provide a life-support system throughout prenatal development.

By the end of the fi rst week, the zygote reaches the uterus. The next step is implan- tation, in which the zygote burrows into the uterine wall and establishes connections with a woman’s blood vessels. Implantation takes about a week to complete and trig- gers hormonal changes that prevent menstruation, letting the woman know that she has conceived.

The implanted zygote is less than a millimeter in diameter, yet its cells have al- ready begun to diff erentiate. A small cluster of cells near the center of the zygote, the germ disc, will eventually develop into the baby. The other cells are destined to become structures that support, nourish, and protect the developing organism. For example, the layer of cells closest to the uterus will become the placenta, a structure through

for their baby. Others argue that this amounts to

eugenics, which is the effort to improve the human

species by allowing only certain people to mate and

pass along their genes to subsequent generations.

Available to all? Most couples who use in

vitro fertilization are in their 30s and 40s, but a

number of older women have begun to use the

technology. Many of these women cannot con-

ceive naturally because they have gone through

menopause and no longer ovulate. Some argue

that it is unfair to a child to have parents who

may not live until the child reaches adulthood.

Others point out that people are living longer

and that middle-aged (or older) adults make bet-

ter parents. (We discuss this issue in more depth

in Chapter 13.)

What do you think? Should prospective par-

ents be allowed to browse a catalog with photos

and biographies of prospective donors? Should

new reproductive technologies be available to all,

regardless of age?

in vitro fertilization

process by which sperm and an egg are

mixed in a petri dish to create a zygote,

which is then placed in a woman’s uterus

eugenics

eff ort to improve the human species by

letting only people whose characteristics

are valued by a society mate and pass

along their genes

zygote

fertilized egg

implantation

step in which the zygote burrows into the

uterine wall and establishes connections

with a woman’s blood vessels

germ disc

small cluster of cells near the center of the

zygote that will eventually develop into a

baby

36 hours after fertilization: 2 cells

5 48 hours after fertilization: 4 cells

6 3 days: A cluster of 16–32 cells7 4 days: A hollow ball of about 100 cells

4–5 days: Zygote enters the uterus

6–7 days: Zygote begins to attach to the wall of the uterus

12–14 days: Zygote is completely implanted in the uterine wall

Cavity of uterus

Inner wall of uterus

Ovary

Fallopian tube leading to uterus

Egg cell divides for the first time

24–30 hours after fertilization male (sperm) and female (egg) chromosome material unite

Fertilization usually takes place in the upper third of the tube, within 24 hours after ovulation

Ovulation: An egg cell from the ovary enters the Fallopian tube at 9–16 days of the menstrual cycle

Figure 2.5 ❚ The period of the zygote spans 14 days, beginning with fertilization of the egg in the Fallopian tube and end-

ing with implantation of the fertilized egg in the wall of the uterus.

placenta

structure through which nutrients and

wastes are exchanged between the mother

and the developing child

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which nutrients and wastes are exchanged between the mother and the developing organism.

Implantation and diff erentiation of cells mark the end of the period of the zy- gote. Comfortably settled in the shelter of the uterus, the zygote is well prepared for the remaining 36 weeks of the marvelous trek leading up to birth.

| Period of the Embryo (Weeks 3–8)

Once the zygote is completely embedded in the uterine wall, it is called an embryo. This new period typically begins the third week after conception and lasts until the end of the eighth week. During the period of the embryo, body structures and internal organs develop. At the beginning of this period, three layers begin to form in the embryo. The outer layer or ectoderm becomes hair, the outer layer of skin, and the nervous system; the middle layer or mesoderm forms muscles, bones, and

the circulatory system; the inner layer or endoderm forms the digestive system and the lungs.

One dramatic way to see these changes is to compare a 3-week-old embryo with an 8-week-old embryo. The 3-week-old embryo is about 2 millimeters long. Specializa- tion of cells is under way, but the organism looks more like a salamander than a hu- man being. However, growth and specialization proceed so rapidly that an 8-week-old embryo looks very diff erent: You can see eyes, jaw, arms, and legs. The brain and the

nervous system are developing rapidly, and the heart has been beating for nearly a month. Most of the organs found in a mature human are in place, in some form. (The sex organs are a notable exception.) Yet because it is only an inch long and weighs but a fraction of an ounce, the embryo is much too small for the mother to feel its presence.

The embryo’s environment is shown in ❚ Figure 2.6. The embryo rests in a sac called the amnion, which is fi lled with amniotic fluid that cushions the embryo and maintains a constant temperature. The embryo is linked to the mother via two struc- tures, the placenta and the umbilical cord. The umbilical cord houses blood vessels that join the embryo to the placenta. In the placenta, the blood vessels from the umbilical cord run close to the mother’s blood vessels but aren’t actually connected to them. The close proximity of the blood vessels allows nutrients, oxygen, vitamins, and waste products to be exchanged between mother and embryo.

Growth in the period of the embryo follows two important principles. First, the head develops before the rest of the body. Such growth, from the head to the base of the spine, illustrates the cephalocaudal principle. Second, arms and legs develop before hands and feet. Growth of parts near the center of the body before those that

are more distant illustrates the proximodistal principle. Growth after birth also fol- lows these principles.

With body structures and internal organs in place, the embryo has passed an- other major milestone in prenatal development. What’s left is for these structures and organs to begin working properly. This is accomplished in the fi nal period of prenatal development, as we’ll see next.

| Period of the Fetus (Weeks 9–38)

The fi nal and longest phase of prenatal development, the period of the fetus, begins at the ninth week (when cartilage begins to turn to bone) and ends at birth. Dur- ing this period, the baby-to-be becomes much larger and its bodily systems begin to work. The increase in size is remarkable. At the beginning of this period, the fetus weighs less than an ounce. At about 4 months, the fetus weighs roughly 4 to 8 ounces, which is large enough for the mother to feel its movements. In the last 5 months of pregnancy, the fetus will gain an additional 7 or 8 pounds before birth.

❚ Figure 2.7, which depicts the fetus at one eighth of its actual size, shows these incred- ible increases in size.

embryo

term given to the zygote once it is com-

pletely embedded in the uterine wall

By the end of the period of the zygote, the fertil-

ized egg has been implanted in the wall of the

uterus and has begun to make connections with

the mother’s blood vessels.

en na

rt N

ils so

At 3 weeks after conception, the fertilized egg

is about 2 millimeters long and resembles a

salamander.

en na

rt N

ils so

At 8 weeks after conception, near the end of the

period of the embryo, the fertilized egg is obvi-

ously recognizable as a baby-to-be.

en na

rt N

ils so

n /

B on

ni er

fo rl

ag en

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During the fetal period, the fi nishing touches are placed on the many systems that are essential to human life, such as respiration, digestion, and vision. Some highlights of this period include the following.

At 4 weeks after conception, a flat set of cells curls to form a tube. One end of ■ the tube swells to form the brain; the rest forms the spinal cord. By the start of the fetal period, the brain has distinct structures and has begun to regulate body functions. During the period of the fetus, all regions of the brain grow— particularly the cerebral cortex, the wrinkled surface of the brain that regu- lates many important human behaviors.

ectoderm

outer layer of the embryo, which will be-

come the hair, the outer layer of skin, and

the nervous system

mesoderm

middle layer of the embryo, which be-

comes the muscles, bones, and circulatory

system

endoderm

inner layer of the embryo, which becomes

the lungs and the digestive system

amnion

inner sac in which the developing child

rests

amniotic fl uid

fl uid that surrounds the fetus

umbilical cord

structure containing veins and arteries

that connects the developing child to the

placenta

cephalocaudal principle

a principle of physical growth that states

that structures nearest the head develop

fi rst

proximodistal principle

principle of physical growth that states

that structures nearest the center of the

body develop fi rst

Amniotic fluid

Placenta

Uterine wall

Uterine wallMother’s blood

Umbilical cord

Amniotic sac Fetus

Blood vessels in umbilical cord

Chorionic villi

Figure 2.6 ❚ The fetus is wrapped in the amniotic sac and

connected to the mother through the

umbilical cord.

9 12 16 20 24 28 32 36 38 Full term

D iff

e re

n ti

a ti

o n

o f

o v a ri

e s

a n

d t

e st

e s

C ir

cu la

to ry

s y st

e m

w o rk

in g

M o v e m

e n

ts f

e lt

b y m

o th

e r

Hair forming

Moore & Persaud, 1993.

Sucking and swallowing

Brain specialization

Age of viability

Rapid weight gain

Birth

Weeks since conception

Figure 2.7 ❚ The baby-to-be becomes much larger during the period of the fetus, and its bodily systems start to work.

W. B. Saunders. Reprinted with permission.

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Near the end of the embryonic period, male embryos develop testes and fe- ■ male embryos develop ovaries. In the 3rd month, the testes in a male fetus secrete a hormone that causes a set of cells to become a penis and scrotum; in a female fetus this hormone is absent, so the same cells become a vagina and labia.

During the 5th and 6th months after conception, eyebrows, eyelashes, and ■ scalp hair emerge. The skin thickens and is covered with a thick greasy sub- stance, or vernix, that protects the fetus during its long bath in amniotic fl uid.

By about 6 months after conception, fetuses differ in their usual heart rates ■ and in how much their heart rate changes in response to physiological stress. In one study (DiPietro et al., 2007), fetuses with greater heart-rate variability were, as 2-month-olds, more advanced in their motor, mental, and language development. Greater heart-rate variability may be a sign that the nervous system is responding efficiently to environmental change (as long as the vari- ability is not extreme).

With these and other rapid changes, by 22 to 28 weeks most systems function well enough that a fetus born at this time has a chance to survive, which is why this age range is called the age of viability. By this age, the fetus has a distinctly baby-like look, but babies born this early have trouble breathing because their lungs are not yet mature.

Also, they cannot regulate their body temperature very well be- cause they lack the insulating layer of fat that appears in the eighth month after conception. With modern neonatal intensive care, in- fants born this early can survive; but they face other challenges, as we’ll see later in this chapter.

During the fetal period, the fetus actually starts to behave (Jo- seph, 2000). The delicate movements that were barely noticeable at 4 months are now obvious. In fact, the fetus is a budding gymnast and kick-boxer rolled into one. It will punch or kick and turn som- ersaults. When active, the fetus moves about once a minute (DiPi- etro et al., 2004). These bursts of activity are followed by times when the fetus is still, as regular activity cycles emerge. Although movement is common in a healthy pregnancy, some fetuses are more active than others and these diff erences predict infants’ be- havior: An active fetus is more likely than an inactive fetus to be

an unhappy, diffi cult baby (DiPietro et al., 1996). Another sign of growing behavioral maturity is that the senses work. There’s not

much to see in the uterus (imagine being in a cave with a fl ashlight that has a weak battery), but there are sounds galore. The fetus can hear the mother’s heart beating and can hear her food digesting. More important, the fetus can hear her speak and can hear others speak to her (Lecanuet, Granier-Deferre, & Busnel, 1995). And there are tastes: As the fetus swallows amniotic fl uid, it responds to diff erent fl avors in the fl uid.

Not only can the fetus detect sounds and fl avors, but sensory experiences from pregnancy can also have lasting eff ects. In one study (Mennella, Jagnow, & Beauchamp, 2001), women drank carrot juice several days a week during the last month of preg- nancy. When their infants were 5 and 6 months old, they preferred cereal with carrot juice. In another study, pregnant women read aloud The Cat in the Hat daily for the last several weeks of pregnancy (DeCasper & Spence, 1986). After birth, the newborns were allowed to suck on a special pacifi er that controlled a tape recorder. The new- borns would suck to hear a tape of their mother reading The Cat in the Hat but not to hear her reading other stories. Evidently, newborns recognized the familiar, rhythmic quality of The Cat in the Hat from their prenatal story times.

Findings like these tell us that the last few months of prenatal development leave the fetus remarkably well prepared for independent living as a newborn baby. Unfor- tunately, not all babies arrive well prepared. Sometimes their prenatal development is disrupted. In the next section, we’ll see how prenatal development can go awry.

period of the fetus

longest period of prenatal development,

extending from the 9th until the 38th

week after conception

cerebral cortex

wrinkled surface of the brain that regu-

lates many functions that are distinctly

human

vernix

substance that protects the fetus’s skin

during development

age of viability

age at which a fetus can survive because

most of its bodily systems function

adequately; typically at 7 months after

conception

At 22–28 weeks after conception, the fetus has

achieved the age of viability, meaning that it has a

chance of surviving if born prematurely.

T H I N K A B O U T I T

Health care professionals often

divide pregnancy into three 3-month

trimesters. How do these three

trimesters correspond to the periods of

the zygote, embryo, and fetus?

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en ce

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C hloe was 2 months pregnant at her fi rst prenatal checkup. As her appointment drew near,

she began a list of questions to ask her obstetrician. “I spend much of my workday at a

computer. Is radiation from the monitor harmful to my baby?” “When my husband and I get

home from work, we’ll have a glass of wine to help unwind from the stress of the day. Is mod-

erate drinking like this okay?” “I’m 38. I know older women give birth to babies with mental

retardation more often. Can I know if my baby will be mentally retarded?”

EACH OF CHLOE’S QUESTIONS CONCERNS HARM TO HER BABY-TO-BE. She worries about the safety of her computer monitor, about her nightly glass of wine, and about her age. Chloe’s concerns are well founded. Many factors infl uence the course of prenatal development, and they are the focus of this section. If you’re sure you can answer all of Chloe’s questions, then skip this section and go directly to page 69. Otherwise, read on to learn about problems that sometimes arise in pregnancy.

| General Risk Factors

As the name implies, general risk factors can have widespread eff ects on prenatal de- velopment. Scientists have identifi ed three general risk factors: nutrition, stress, and a mother’s age.

Test Yourself

RECALL

1. The period of the zygote ends

2. Body structures and internal organs are created during the

period of the .

3. is called the age of viability because

this is when most body systems function well enough to

support life.

4. In the last few months of prenatal development, the fetus

has regular periods of activity and ,

which are the fi rst signs of fetal behavior.

INTERPRET

Compare the events of prenatal development that precede the

age of viability with those that follow it.

APPLY

In the last few months before birth, the fetus has some basic

perceptual and motor skills; a fetus can hear, see, taste, and

move. What are the advantages of having these skills in place

months before they’re really needed?

Recall answers: (1) at 2 weeks after conception (when the zygote is completely implanted

in the wall of the uterus), (2) embryo, (3) Between 22 and 28 weeks, (4) the eyes and ears

respond to stimulation

2.3 INFLUENCES ON PRENATAL DEVELOPMENT

L E A R N I N G O B J E C T I V E S

How is prenatal development influenced by a pregnant wom- ❚ an’s age, her nutrition, and the stress she experiences while

pregnant?

How do diseases, drugs, and environmental hazards some- ❚ times affect prenatal development?

What general principles affect the ways that prenatal devel- ❚ opment can be harmed?

How can prenatal development be monitored? Can abnormal ❚ prenatal development be corrected?

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Nutrition The mother is the developing child’s sole source of nutrition, so a balanced diet that includes foods from each of the fi ve major food groups is vital. Most pregnant women need to increase their intake of calories by about 10 to 20% to meet the needs of prenatal development. A woman should expect to gain between 25 and 35 pounds during pregnancy, assuming that her weight was normal before pregnancy. A woman who was underweight before becoming pregnant may gain as much as 40 pounds; a woman who was overweight should gain at least 15 pounds (Institute of Medicine, 1990). Of this gain, about one third refl ects the weight of the baby, the placenta, and the fl uid in the amniotic sac; another third comes from increases in a woman’s fat stores; and yet another third comes from the increased volume of blood and increases in the size of the woman’s breasts and uterus (Whitney & Hamilton, 1987).

Sheer amount of food is only part of the equation for a healthy pregnancy. What a pregnant woman eats is also very important. Proteins, vitamins, and minerals are essential for normal prenatal development. For example, folic acid (one of the B vi- tamins) is important for the baby’s nervous system to develop properly (Shaw et al., 1995). When mothers do not consume adequate amounts of folic acid, their babies are at risk for spina bifida, a disorder in which the embryo’s neural tube does not close properly during the fi rst month of pregnancy. Since the neural tube develops into the brain and spinal cord, the result when it does not close properly is permanent dam- age to the spinal cord and the nervous system. Many children with spina bifi da need crutches, braces, or wheelchairs. Other prenatal problems have also been traced to inadequate proteins, vitamins, or minerals, so health care providers typically recom- mend that pregnant women supplement their diet with additional proteins, vitamins, and minerals.

When a pregnant woman does not provide adequate nourishment, the infant is likely to be born prematurely and to be underweight. Inadequate nourishment during the last few months of pregnancy can particularly aff ect the nervous system, because this is a time of rapid brain growth. Finally, babies who do not receive adequate nour- ishment are vulnerable to illness (Morgane et al., 1993).

Stress Does a pregnant woman’s mood aff ect the zygote, embryo, or fetus in her uterus? Is a woman who is happy during pregnancy more likely to give birth to a happy baby? Is a harried offi ce worker more likely to give birth to an irritable baby? These questions address the impact on prenatal development of chronic stress, which re- fers to a person’s physical and psychological responses to threatening or challeng- ing situations. We can answer these questions with some certainty for nonhumans. When pregnant female animals experience constant stress—such as repeated elec- tric shocks or intense overcrowding—their off spring are often smaller than average and prone to other physical and behavioral problems (DiPietro, 2004). In addition, stress seems to cause greater harm when experienced early in pregnancy (Schneider et al., 1999).

Determining the impact of stress on human pregnancy is more diffi cult because we must rely solely on correlational studies. (It would be unethical to do an ex- periment that assigned some pregnant women to a condition of extreme stress.) Studies typically show that women who report greater anxiety during pregnancy more often give birth early or have babies who weigh less than average (Copper et al., 1996; Paarlberg et al., 1995). What’s more, when women are anxious through- out pregnancy, their children are less able to pay attention as infants and more prone to behavioral problems as preschoolers (Huizink et al., 2002; O’Conner et al., 2002). Similar results emerges in studies of pregnant women exposed to disasters,

such as the September 11 attacks on the World Trade Center: their children’s physi- cal and behavioral development is aff ected (Engel et al., 2005; Laplante et al., 2004). Finally, the harmful eff ects of stress may be particularly evident when women are anxious about their pregnancy per se and not simply anxious in general (DiPietro et al., 2006).

spina bifi da

disorder in which the embryo’s neural

tube does not close properly

stress

physical and psychological responses to

threatening or challenging conditions

When pregnant women experience chronic

stress, they’re more likely to give birth early or

have smaller babies; this may be because women

who are stressed are more likely to smoke or

drink and less likely to rest, exercise, and eat

properly.

ub bl

es P

ho to

lib ra

ry /

A la

m y

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Increased stress can harm prenatal development in several ways. First, when a pregnant woman experiences stress, her body secretes hormones that reduce the fl ow of oxygen to the fetus while increasing its heart rate and activity level (Monk et al., 2000). Second, stress can weaken a pregnant woman’s immune system, making her more susceptible to illness (Cohen & Williamson, 1991) that can, in turn, damage fetal development. Third, pregnant women under stress are more likely to smoke or drink alcohol and are less likely to rest, exercise, and eat properly (DiPietro et al., 2004). All these behaviors endanger prenatal development.

We want to emphasize that the results described here apply to women who expe- rience prolonged, extreme stress. Virtually all women sometimes become anxious or upset while pregnant. Occasional, relatively mild anxiety is not thought to have any harmful consequences for prenatal development.

Mother’s Age Traditionally, the 20s were thought to be the prime childbearing years. Teenage women as well as women who were 30 or older were considered less fi t for the rigors of preg- nancy. Is being a 20-something really important for a successful pregnancy? Let’s an- swer this question separately for teenage and older women. Compared to women in their 20s, teenage women are more likely to have problems during pregnancy, labor, and delivery. This is largely because pregnant teenagers are more likely to be economi- cally disadvantaged and to lack good prenatal care—either because they are unaware of the need for it or because they cannot aff ord it. For example, in one study (Turley, 2003) children of teenage moms were compared with their cousins, whose mothers were the older sisters of the teenage moms but had given birth when they were in their 20s. The two groups of children were similar in academic skills and behavioral problems, indicating that it’s not the age but rather the typical family background of teenage moms that creates problems. Similarly, research done on African Ameri- can adolescents indicates that, when diff erences in prenatal care are taken into account, teenagers are just as likely as women in their 20s to have problem-free pregnancies and to give birth to healthy babies (Goldenberg & Klerman, 1995).

Nevertheless, even when a teenager receives adequate pre- natal care and gives birth to a healthy baby, all is not rosy. Chil- dren of teenage mothers generally do less well in school and more often have behavioral problems (Fergusson & Woodward, 2000). The problems of teenage motherhood—incomplete edu- cation, poverty, and marital diffi culties—aff ect the child’s later development (Moore & Brooks-Gunn, 2002).

Of course, not all teenage mothers and their infants follow this dismal life course. Some teenage mothers fi nish school, fi nd good jobs, and have happy marriages; their children do well in school, academically and socially. These “success stories” are more likely when teenage moms live with a relative—typically, the child’s grandmother (Gordon, Chase-Lansdale, & Brooks-Gunn, 2004). However, teenage pregnancies with “happy endings” are defi nitely the exception; for most teen- age mothers and their children, life is a struggle. Educating teenagers about the true consequences of teen pregnancy is crucial.

Are older women better suited for pregnancy? This is an important question be- cause today’s American woman is waiting longer than ever to have her fi rst child. Completing an education and beginning a career often delay childbearing. In fact, the birth rate in the early 2000s among 30- to 44-year-olds was nearly double what it was in 1980 (Martin et al., 2002).

Traditionally, older women were thought to have more diffi cult pregnancies and more complicated labor and deliveries. Today, we know that women in their 20s are twice as fertile as women in their 30s (Dunson et al., 2002). For women 35 years of age and older, the risks of miscarriage and stillbirth increase rapidly. Among 40- to 45-year-olds, for example, nearly half of all pregnancies result in miscarriage (Andersen

For teenage mothers and their babies, life is

often a struggle because the mothers are unable

to complete their education and often live in

poverty.

ild er

lo un

ge /

A la

m y

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et al., 2000). Also, women in their 40s are more liable to give birth to babies with Down syndrome. As mothers, however, older women are quite eff ective. For example, they are just as able to provide the sort of sensitive, responsive caregiving that promotes a child’s development (Bornstein et al., 2006).

In general, then, prenatal development is most likely to proceed normally when women are between 20 and 35 years of age, are healthy and eat right, get good health care, and lead lives that are free of chronic stress. But even in these optimal cases, pre- natal development can be disrupted, as we’ll see in the next section.

| Teratogens: Drugs, Diseases, and Environmental Hazards

In the late 1950s, many pregnant women in Germany took thalidomide, a drug that helped them sleep. Soon, however, came reports that many of these women were giving birth to babies with deformed arms, legs, hands, or fi ngers. Thalidomide is a powerful teratogen, an agent that causes abnormal prenatal development. Ultimately, more than 7,000 babies worldwide were harmed before thalidomide was withdrawn from the market (Kolberg, 1999).

Prompted by the thalidomide disaster, scientists began to study teratogens exten- sively. Today, we know a great deal about many teratogens that aff ect prenatal devel- opment. Most teratogens fall into one of three categories: drugs, diseases, or environ- mental hazards. Let’s look at each.

Drugs Thalidomide illustrates the harm that drugs can cause during prenatal development. ● Table 2.3 lists several other drugs that are known teratogens. Most of the drugs in the list are substances you may use routinely—alcohol, aspirin, caff eine, nicotine. Nevertheless, when consumed by pregnant women, they do present special dangers (Behnke & Eyler, 1993).

Cigarette smoking is typical of the potential harm from teratogenic drugs (Cornelius et al., 1995; Fried, O’Connell, & Watkinson, 1992). The nicotine in cigarette smoke constricts blood vessels and thus reduces the oxygen and nutrients that can reach the fetus over the placenta. Therefore, pregnant women who smoke are more likely to miscarry (abort the fetus spontaneously) and to bear children who are smaller than average at birth (Cnattingius, 2004; Ernst, Moolchan, & Robinson, 2001). And, as children develop, they are more likely to show signs of impaired attention, language, and cognitive skills as well as behavioral problems (Brennan et al., 2002; Wakschlag et al., 2006). Finally, even secondhand smoke harms the fetus: When pregnant women don’t smoke but fathers do, babies tend to be smaller at birth (Friedman & Polifka, 1996). The message is clear and simple: Pregnant women shouldn’t smoke, and they should avoid others who do smoke.

Alcohol also carries serious risk. Pregnant women who consume large quantities of alcoholic beverages often give birth to babies with fetal alcohol syndrome (FAS). Children with FAS usually grow more slowly than normal and have heart problems and misshapen faces. Youngsters with FAS often have a small head, a thin upper lip,

teratogen

an agent that causes abnormal prenatal

development

● TA B L E 2 . 3

Teratogenic Drugs and Their Consequences

Drug Potential Consequences

Alcohol Fetal alcohol syndrome, cognitive deficits, heart damage, retarded growth

Aspirin Deficits in intelligence, attention, and motor skills

Caffeine Lower birth weight, decreased muscle tone

Cocaine and heroin Retarded growth, irritability in newborns

Marijuana Lower birth weight, less motor control

Nicotine Retarded growth, possible cognitive impairments

fetal alcohol syndrome (FAS)

disorder aff ecting babies whose mothers

consumed large amounts of alcohol while

they were pregnant

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a short nose, and widely spaced eyes. FAS is the leading cause of mental retardation in the United States, and children with FAS have serious attentional, cognitive, and behavioral problems (see, e.g., Howell et al., 2006). Fetal alcohol syndrome is most common among pregnant women who are heavy recreational drinkers (Jacobson & Jacobson, 2000; Lee, Mattson, & Riley, 2004).

Fetal alcohol syndrome is most likely when pregnant women drink 5 ounces (or more) of alcohol daily. Does this mean that moderate drinking is safe? No. When women drink moderately throughout pregnancy, their children often have lower scores on tests of attention, memory, and intelligence (Streissguth et al., 1994).

Is there any amount of drinking that’s safe during pregnancy? Maybe, but scien- tists have yet to determine one. This inconclusiveness stems from two factors. First, drinking is often estimated from women’s responses to interviews or questionnaires. These replies may be incorrect, leading to inaccurate estimates of the harm associated with drinking. Second, any safe level of consumption is probably not the same for all women. Based on their health and heredity, some women may be able to consume more alcohol safely than others.

These factors make it impossible to off er guaranteed statements about safe levels of alcohol or any of the other drugs listed in Table 2.3. For this reason, the best policy is for women to avoid all drugs throughout pregnancy.

Diseases Sometimes women become ill while pregnant. Most diseases, such as colds and many strains of the fl u, do not aff ect the fetus. However, several bacterial and viral infections can be quite harmful; fi ve are listed in ● Table 2.4.

Some diseases pass from the mother through the placenta to attack the embryo or fetus directly. AIDS, cytomegalovirus, rubella, and syphilis are examples of diseases that are transmitted through the placenta. Other diseases attack during birth: The virus is present in the lining of the birth canal, and babies are infected as they pass through the canal. AIDS and genital herpes are two such diseases.

The only way to guarantee that these diseases will not harm prenatal development is for a woman to be sure that she does not contract them either before or during her pregnancy. Medicines that may help to treat a woman after she has become ill do not prevent the disease from damaging the fetus.

T H I N K A B O U T I T

A pregnant woman reluctant to give up

her morning cup of coffee and nightly

glass of wine says, “I drink so little coffee

and wine that it couldn’t possibly hurt

my baby.” What do you think?

When pregnant women drink large amounts

of alcohol, their children often have fetal alco-

hol syndrome; they tend to have a small head

and a thin upper lip as well as retarded mental

development.

K . L

. J on

es /L

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rc h

● TA B L E 2 . 4

Teratogenic Diseases and Their Consequences

Disease Potential Consequences

AIDS Frequent infections, neurological disorders, death

Cytomegalovirus Deafness, blindness, abnormally small head, mental retardation

Genital herpes Encephalitis, enlarged spleen, improper blood clotting

Rubella (German measles) Mental retardation; damage to eyes, ears, and heart

Syphilis Damage to the central nervous system, teeth, and bones

Environmental Hazards As a by-product of life in an industrialized world, people are often exposed to toxins in food they eat, fl uids they drink, and air they breathe. Chemicals associated with in- dustrial waste are the most common form of environmental teratogens. The quantity involved is usually minute; however, as with drugs, amounts that go unnoticed in an adult can cause serious damage to the fetus.

Polychlorinated biphenyls (PCBs) illustrate the danger of environmental terato- gens. These were used in electrical transformers and paints until the U.S. government banned them in the 1970s. However, PCBs (like many industrial by-products) seeped into the waterways, where they contaminated fi sh and wildlife. The amount of PCBs in a typical contaminated fi sh does not aff ect adults, but when pregnant women ate

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Environmental teratogens are treacherous because people are unaware of their presence in the environment. For example, the women studied by Jacobson, Jacob- son, and Humphrey (1990) did not realize they were eating PCB-laden fi sh. Thus, the invisibility of some environmental teratogens makes it more diffi cult for a pregnant woman to protect herself from them. The best advice is for a pregnant woman to be particularly careful about the foods she eats and the air she breathes. Be sure that all foods are cleaned thoroughly to rid them of insecticides. Try to avoid convenience foods, which often contain many chemical additives. Stay away from air that’s been contaminated by household products such as cleansers, paint strippers, and fertilizers. Women in jobs (such as housecleaning or hairdressing) that require contact with po- tential teratogens should switch to less potent chemicals, if possible. For example, they should use baking soda instead of more chemically laden cleansers; and they should wear protective gloves, aprons, and masks to reduce their contact with potential terato- gens. Finally, because environmental teratogens continue to increase, check with a health care provider to learn whether other materials should be avoided.

| How Teratogens Influence Prenatal Development

By assembling all the evidence on the harm caused by drugs, diseases, and environ- mental hazards, scientists have identifi ed fi ve important general principles about how teratogens usually work (Hogge, 1990; Jacobson & Jacobson, 2000; Vorhees & Mollnow, 1987).

1. The impact of a teratogen depends on the genotype of the organism. A substance may be harmful to one species but not to another. To deter- mine its safety, thalidomide was tested on pregnant rats and rabbits, and their offspring had normal limbs. Yet when pregnant women took the same drug in comparable doses, many had children with deformed limbs. More- over, some women who took thalidomide gave birth to babies with normal limbs whereas others, taking comparable doses of thalidomide at the same time in their pregnancies, gave birth to babies with deformed arms and legs. Apparently, heredity makes some individuals more susceptible than others to a teratogen.

2. The impact of teratogens changes over the course of prenatal development. The timing of exposure to a teratogen is very important. Teratogens typi-

● TA B L E 2 . 5

Environmental Teratogens and Their Consequences

Hazard Potential Consequences

Lead Mental retardation

Mercury Retarded growth, mental retardation, cerebral palsy

PCBs Impaired memory and verbal skills

X-rays Retarded growth, leukemia, mental retardation

large numbers of PCB-contaminated fi sh, their children’s cognitive skills and reading achievement were impaired (Jacobson & Jacobson, 1996).

Several environmental hazards that are known teratogens are listed in ● Table 2.5. You’ll notice that although X-rays are included in the table, radiation associated with computer monitors or video-display terminals (VDTs) is not. Several major stud- ies have examined the impact of exposure to the electromagnetic fi elds generated by VDTs. For example, Schnorr and her colleagues (1991) compared the outcomes of pregnancies in telephone operators who worked at VDTs at least 25 hours weekly with those of telephone operators who never used VDTs. For both groups of women, about 15% of the pregnancies ended in miscarriage. Further, other studies have not found links between exposure to VDTs and birth defects (Parazzini et al., 1993). Evidently, VDTs can be used safely by pregnant women.

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cally have diff erent eff ects in the three periods of prenatal development. ❚ Figure 2.8 shows how the consequences of teratogens diff er for the pe- riods of the zygote, embryo, and fetus. During the period of the zygote, exposure to teratogens usually results in spontaneous abortion of the fertil- ized egg. During the period of the embryo, exposure to teratogens produces major defects in bodily structure. For instance, women who took thalido- mide during the period of the embryo had babies with ill-formed or missing limbs, and women who contract rubella during the period of the embryo have babies with heart defects. During the period of the fetus, exposure to teratogens either produces minor defects in bodily structure or causes body systems to function improperly. For example, when women drink large quantities of alcohol during this period, the fetus develops fewer brain cells.

Even within the diff erent periods of prenatal development, developing body parts and systems are more vulnerable at some times than others. The red shading in the chart indicates a time of maximum vulnerability; orange shading indicates a time when the developing organism is less vulnerable. The heart, for example, is most sensitive to teratogens during the fi rst half of the embryonic period. Exposure to teratogens before this time rarely pro- duces heart damage, and exposure after this time results in relatively mild damage.

3. Each teratogen aff ects a specifi c aspect (or aspects) of prenatal development. Said another way, teratogens do not harm all body systems; instead, damage is selective. When women contract rubella, their babies often have prob- lems with their eyes, ears, and heart but have normal limbs. When mothers

Age of embryo (in weeks) Fetal period (in weeks)

Prenatal death Major congenital anomalies (red)

1 2 3 4 5 6 7 8 9 16 3820-36

Period of dividing zygote, implantation,

and bilaminar embryo

Heart

Upper limbs

Eyes

Lower limbs

Functional defects and minor congenital anomalies (orange)

Full term

C.N.S.

Heart HeartEye Eye Ear

Limbs

Ear

Teeth

Palate

External genitalia

Brain

Central nervous system

Teeth

Palate

External genitalia

Ears

Not susceptible to

teratogens

Moore and Persaud, 1993.

Indicates common site of action of teratogen

Figure 2.8 ❚ The effects of a teratogen on an unborn child depend on the stage of prenatal development.

permission.

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consume PCB-contaminated fi sh, their babies typically have normal body parts and normal motor skill but below-average verbal and memory skills.

4. The impact of teratogens depends on the dose. Just as a single drop of oil won’t pollute a lake, small doses of teratogens may not harm the fetus. In research on PCBs, for example, cognitive skills were aff ected only among children who had the greatest prenatal exposure to these by-products. In general, the greater the exposure, the greater the risk for damage (Adams, 1999). An implication of this principle is that researchers should be able to determine safe levels for a teratogen. In reality, this is extremely diffi cult because sensitivity to teratogens is not the same for all people (and it’s not practical to establish separate safe amounts for each person). Hence, the safest rule is zero exposure to teratogens.

5. Damage from teratogens is not always evident at birth but may appear later in life. In the case of malformed limbs or babies born addicted to cocaine, the eff ects of a teratogen are obvious immediately. Sometimes, however, the damage from a teratogen becomes evident only as the child develops. For example, when women ate PCB-contaminated fi sh, their babies were normal at birth. Their below-average cognitive skills were not evident until several months later.

An even more dramatic example of the delayed impact of a teratogen in- volves the drug diethylstilbestrol (DES). Between 1947 and 1971, many preg- nant women took DES to prevent miscarriages. Their babies were apparently normal at birth. As adults, however, daughters of women who took DES are more likely to have a rare cancer of the vagina and to have difficulties becom- ing pregnant themselves. Sons of women who took DES may be less fertile and at risk for cancer of the testes (Sharpe & Skakkebaek, 1993). In this case, the impact of the teratogen is not evident until decades after birth.

The Real World of Prenatal Risk We have discussed risk factors individually as if each factor were the only potential threat to prenatal development. In reality, many infants are exposed to multiple gen- eral risks and multiple teratogens. Pregnant women who drink alcohol often smoke and drink coff ee (Haslam & Lawrence, 2004). Pregnant women who are under stress often drink alcohol (Giberson & Weinberg, 1992). Many of these same women may have poor nutrition. When all of the risks are combined, prenatal development will rarely be optimal (Schneider, Roughton, & Lubach, 1997).

This pattern explains why it’s often challenging for human development research- ers to determine the harm associated with individual teratogens. Cocaine is a perfect example. You may remember stories in newspapers and magazines about “crack ba- bies” and their developmental problems. In fact, the jury is still out on the issue of co- caine as a teratogen (Jones, 2006). Some investigators (e.g., Dennis et al., 2006; Singer et al., 2002) fi nd the harmful eff ects that made headlines, but others (e.g., Brown et al., 2004; Frank et al., 2001) argue that most of the eff ects attributed to cocaine actually stem from concurrent smoking and drinking and to the inadequate parenting that these children receive.

From what we’ve said so far in this section, you may think that the developing child has little chance of escaping harm. But most babies are born in good health. Of course, a good policy for pregnant women is to avoid diseases, drugs, and environmen- tal hazards that are known teratogens. This, coupled with thorough prenatal medical care and adequate nutrition, is the best recipe for normal prenatal development.

| Prenatal Diagnosis and Treatment

“I really don’t care whether I have a boy or girl, just as long as it’s healthy.” Legions of parents worldwide have felt this way, but until recently, all they could do was hope for the best. Today, however, advances in technology mean that parents can have a much better idea of whether their baby is developing normally.

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Genetic Counseling Often the fi rst step in deciding whether a couple’s baby is likely to be at risk is genetic counseling. A counselor asks about family medical history and constructs a family tree for each parent to assess the odds that their child would inherit a disorder. If the family tree suggests that a parent is likely to be a carrier of the disorder, blood tests can determine the parent’s genotype. With this information, a genetic counselor then advises prospective parents about their choices. A couple might simply go ahead and attempt to conceive a child “naturally.” Alternatively, they may decide to use sperm or eggs from other people. Yet another choice might be adoption.

Prenatal Diagnosis After a woman is pregnant, how can we know whether prenatal development is progressing normally? Traditionally, obstetricians tracked the prog- ress of prenatal development by feeling the size and position of the fetus through a woman’s abdomen. This technique was not very precise and, of course, couldn’t be done at all until the fetus was large enough to feel. Today, however, several new tech- niques have revolutionized our ability to monitor prenatal growth and development. A standard part of prenatal care in-the United States is ultrasound, in which sound waves are used to generate a picture of the fetus. In this procedure, a tool about the size of a hair dryer is rubbed over the woman’s abdomen, and the image appears on a nearby computer monitor. The pictures generated are hardly portrait quality; they are grainy, and it takes an expert’s eye to distinguish what’s what. Nevertheless, parents are often thrilled to see their baby and to watch it move.

Ultrasound typically can be used as early as 4 or 5 weeks after conception; prior to this time, the embryo is not large enough to generate an interpretable image. Ul- trasound pictures are quite useful for determining the position of the fetus within the uterus and, at 16–20 weeks after conception, its sex. Ultrasound is also helpful in detecting twins or triplets. Finally, ultrasound is used to identify gross physical defor- mities, such as abnormal growth of the head.

In pregnancies where a genetic disorder is suspected, two other techniques are par- ticularly valuable because they provide a sample of fetal cells that can be analyzed. In amniocentesis, a needle is inserted through the mother’s abdomen to obtain a sample of the amniotic fl uid that surrounds the fetus. As you can see in ❚ Figure 2.9, ultrasound is

ultrasound

prenatal diagnostic technique that uses

sound waves to generate an image of the

fetus

amniocentesis

prenatal diagnostic technique that uses a

syringe to withdraw a sample of amniotic

fl uid through the mother’s abdomen

Uterine wall

Placenta

Ultrasound scanner

Figure 2.9 ❚ In amniocentesis, a sample of fetal cells is ex-

tracted from the fluid in the amniotic sac.

A standard part of prenatal care is ultrasound,

in which sound waves are used to generate an

image of the fetus that can be used to determine

its position in the uterus.

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used to guide the needle into the uterus. The fl uid contains skin cells that can be grown in a laboratory dish and then analyzed to determine the genotype of the fetus.

A procedure that can be used much earlier in pregnancy is chorionic villus sampling (CVS) in which a sample of tissue is obtained from part of the placenta. ❚ Figure 2.10 shows that a small tube—typically inserted through the vagina and into the uterus but sometimes through the abdomen—is used to collect a small plug of cells from the placenta. This procedure can be used within 10 or 12 weeks after concep- tion, much earlier than amniocentesis.

Results are returned from the lab in about two weeks following amniocentesis and in 7–10 days following CVS. (The wait is longer for amniocentesis because genetic material can’t be evaluated until enough cells have reproduced for analysis.) With the samples obtained from either technique, roughly 200 diff erent genetic disorders, including Down syndrome, can be detected. These procedures are virtually free of er- rors but come at a price: Miscarriages are slightly—1 or 2%—more likely after amnio- centesis or chorionic villus sampling (Wilson, 2000). A woman must decide whether the information gained from amniocentesis or chorionic villus sampling justifi es the slightly increased risks of a possible miscarriage.

Fetal Medicine Ultrasound, amniocentesis, and chorionic villus sampling have made it much easier to determine whether prenatal development is progressing normally. But what hap- pens when it is not? Traditionally, a woman’s options have been limited: She could continue the pregnancy or end it. Today the list of options is expanding. A whole new fi eld called fetal medicine is concerned with treating prenatal problems before birth. Many tools are now available to solve problems that are detected during pregnancy (Evans, Platt, & De La Cruz, 2001). One approach is to treat disorders medically by administering drugs or hormones to the fetus. For example, in fetal hypothyroidism, the fetal thyroid gland does not produce enough hormones. This can lead to retarded physical and mental development, but the disorder can be treated by injecting the necessary hormones directly into the amniotic cavity, resulting in normal growth. Another example is congenital adrenal hyperplasia, an inherited disorder in which the fetal adrenal glands produce too much androgen; this causes early maturation of boys or masculinization of girls. Treatment consists of injecting hormones into the mother that reduce the amount of androgen secreted by the fetal adrenal glands (Evans et al., 2001).

Ultrasound scanner

Chorionic villi

Uterine wall

Vagina

Figure 2.10 ❚ In chorionic villus sampling, fetal cells are ex-

tracted from the placenta.

chorionic villus sampling (CVS)

prenatal diagnostic technique that

involves taking a sample of tissue from

the chorion

fetal medicine

fi eld of medicine concerned with treating

prenatal problems before birth

T H I N K A B O U T I T

Imagine that you are 42 years old and

pregnant. Would you want to have

amniocentesis or chorionic villus

sampling to determine the genotype of

the fetus? Why or why not?

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Another way to correct prenatal problems is fetal surgery (Warner, Altimier, & Crombleholme, 2007). For instance, spina bifi da can be corrected with fetal surgery in the seventh or eighth month of pregnancy. Surgeons cut through the mother’s ab- dominal wall to expose the fetus and then cut through the fetal abdominal wall; the spinal cord is repaired, and the fetus is returned to the uterus (Okie, 2000).

Fetal surgery has also been used to treat a disorder aff ecting identical twins in which one twin—the “donor”—pumps blood through its own and the other twin’s circulatory system. The donor twin usually fails to grow, but surgery can correct the problem by sealing off the unnecessary blood vessels between the twins (Danzer et al., 2003). Fetal surgery holds great promise, but it is still highly experimental and thus is viewed as a last resort.

Yet another approach is genetic engineering, in which defective genes are replaced by synthetic normal genes. Take sickle-cell disease as an example. Recall from page 44 that, when a baby inherits the recessive allele for sickle-cell disease from both parents, the child has misshaped red blood cells that can’t pass through capillaries. In theory, it should be possible to take a sample of cells from the fetus, remove the recessive genes from the 11th pair of chromosomes, and replace them with the dominant genes. These “repaired” cells could then be injected into the fetus, where they would multiply and cause normal red blood cells to be produced (Verma, 1990).

Translating this idea into practice has been diffi cult, and there are many problems yet to be solved (Cooke, 2005). Nevertheless, gene therapy has been successful in a few cases. In one, a preschool girl was suff ering from a hereditary disease of the immune system that left her unprotected against infection. Doctors took some of the girl’s cells and inserted the immune gene into them. The cells were then injected into her blood- stream, where they help ward off infection.

These techniques are highly experimental, and failures are common. However, this area of medicine is advancing rapidly, and prenatal treatment should become much more common in the 21st century.

Answers to Chloe’s Questions. Now you can return to Chloe’s questions in the sec- tion-opening vignette (page 59) and answer them for her. If you’re not certain, here are the pages in this chapter where the answers appear:

About her computer monitor—page 64 ■

About her nightly glass of wine—page 63 ■

About giving birth to a baby with mental retardation—page 62 ■

Recall answers: (1) prolonged stress, (2) Environmental hazards, (3) spontaneous abortion of

the fertilized egg, (4) chorionic villus sampling

Test Yourself

RECALL

1. General risk factors in pregnancy

include a woman’s nutrition,

, and her age.

2. are some of the most dangerous

teratogens because a pregnant woman is often unaware of

their presence.

3. During the period of the zygote, exposure to a teratogen

typically results in .

4. Two techniques used to determine whether a fetus

has a hereditary disorder are amniocentesis and

INTERPRET

Explain how the impact of a teratogen changes over the

course of prenatal development.

APPLY

What would you say to a 45-year-old woman who is eager to

become pregnant but is unsure about the possible risks associ-

ated with pregnancy at this age?

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M arlea is about to begin classes to prepare for her baby’s birth. She is relieved that the

classes are fi nally starting because this means the end of pregnancy is in sight. But all the

talk she has heard about “breathing exercises” and “coaching” sounds pretty silly to her. Marlea

would prefer to get knocked out for the delivery and wake up when everything is over.

AS WOMEN LIKE MARLEA NEAR THE END OF PREGNANCY, they fi nd that sleeping and breathing become more diffi cult, that they tire more rapidly, that they become consti- pated, and that their legs and feet swell. Women look forward to birth, both to relieve their discomfort and, of course, to see their baby. In this section, you’ll see the diff erent steps involved in birth, review diff erent approaches to childbirth, and look at prob- lems that can arise. Along the way, we’ll look at classes like those Marlea will take and the exercises that she’ll learn.

| Stages of Labor

Labor is an appropriate name for childbirth, which is the most intense, prolonged physical eff ort that humans experience. Labor is usually divided into the three stages shown in ❚ Figure 2.11.

In stage 1, which may last from 12 to 24 hours for a first birth, the uterus ■ starts to contract. The first contractions are weak and irregular. Gradually, they become stronger and more rhythmic, enlarging the cervix (the opening from the uterus to the vagina) to approximately 10 centimeters.

In stage 2, the baby passes through the cervix and enters the vagina. The ■ mother helps push the baby along by contracting muscles in her abdomen. Soon the top of the baby’s head appears, an event known as crowning. Within about an hour, the baby is delivered.

In stage 3, which lasts only minutes, the mother pushes a few more times to ■ expel the placenta (also called, appropriately, the afterbirth).

L E A R N I N G O B J E C T I V E S

What are the different phases of labor and delivery? ❚

What are “natural” ways of coping with the pain of child- ❚ birth? Is childbirth at home safe?

What adjustments do parents face after a baby’s birth? ❚

What are some complications that can occur during birth? ❚

What contributes to infant mortality in developed and least ❚ developed countries?

2.4 LABOR AND DELIVERY

Umbilical cord

Dilated cervix

Detached placenta

Stage 1 Stage 2 Stage 3

Figure 2.11 ❚ Labor includes three stages, beginning when

the uterus contracts and ending when the

placenta is expelled.

crowning

appearance of the top of the baby’s head

during labor

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The times given for each of the stages are only approximations; the actual times vary greatly among women. For most women, labor with their second and subsequent chil- dren is much more rapid. Stage 1 may last 4 to 6 hours, and stage 2 may be as brief as 20 minutes.

| Approaches to Childbirth

When your authors were born in the 1950s, women in labor were admitted to a hospi- tal and administered a general anesthetic. Fathers waited anxiously in a nearby room for news of the baby. These were standard hospital procedures and virtually all Ameri- can babies were born this way.

But no longer is this true. In the middle of the 20th century, two European physicians—Grantly Dick-Read (1959) and Ferdinand Lamaze (1958)—criticized the traditional view in which labor and delivery had come to involve elaborate medi- cal procedures that were often unnecessary and that often left women afraid of giving birth. This fear led them to be tense, thereby increasing the pain they ex- perienced during labor. These physicians argued for a more “natural” or prepared approach to childbirth, viewing labor and delivery as life events to be celebrated rather than medical procedures to be endured.

Today many varieties of prepared childbirth are available to pregnant women. However, most share some fundamental beliefs. One is that birth is more likely to be problem-free and rewarding when mothers and fathers understand what’s happening during pregnancy, labor, and delivery. Consequently, prepared child- birth means going to classes to learn basic facts about pregnancy and childbirth (like the material presented in this chapter).

A second common element is that natural methods of dealing with pain are emphasized over medication. Why? When a woman is anesthetized with ei- ther general anesthesia or regional anesthesia (in which only the lower body is numbed), she can’t use her abdominal muscles to help push the baby through the birth canal. Without this pushing, the obstetrician may have to use mechanical devices to pull the baby through the birth canal, which involves some risk to the baby (Johanson et al., 1993). Also, drugs that reduce the pain of childbirth cross the placenta and can aff ect the baby. Consequently, when a woman receives large doses of pain-relieving medication, her baby is often withdrawn or irritable for days or even weeks (Brazelton, Nugent, & Lester, 1987; Ransjoe-Arvidson et al., 2001). These eff ects are temporary, but they may give the new mother the impres- sion that she has a diffi cult baby. It is best, therefore, to minimize the use of pain- relieving drugs during birth.

Relaxation is the key to reducing birth pain without drugs. Because pain often feels greater when a person is tense, pregnant women learn to relax during labor, through deep breathing or by visualizing a reassuring, pleasant scene or experience. Whenever they begin to experience pain during labor, they use these methods to relax.

A third common element of prepared childbirth is to involve a supportive “coach.” The father-to-be, a relative, or a close friend attends childbirth classes with the moth- er-to-be. The coach learns the techniques for coping with pain and practices them with the pregnant woman. During labor and delivery, the coach is present to help the woman use the techniques she has learned and to off er support and encouragement. Sometimes the coach is accompanied by a doula, a person familiar with childbirth who is not part of the medical staff but provides emotional and physical support throughout labor and delivery.

Although Marlea, the pregnant woman in the vignette, may have her doubts about these classes, research shows that they are useful (Hetherington, 1990). Most mothers who attend childbirth classes use some medication to reduce the pain of labor, but they typically use less than mothers who do not attend childbirth classes. Also, mothers and fathers who attend childbirth classes feel more positively about labor and birth when compared to mothers and fathers who have not attended classes.

During childbirth preparation classes, pregnant

women learn exercises that help them relax and

reduce the pain associated with childbirth.

doula

person familiar with childbirth who

provides emotional and physical support

throughout labor and delivery

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Another element of the trend to natural childbirth is the idea that birth need not always take place in a hospital. Virtu- ally all babies in the United States are born in hospitals, with only 1% born at home (Curtain & Park, 1999). Yet around the world—in Europe, South America, and Asia—many children are born at home, refl ecting a cultural view that the best place to welcome a new family member is at home, surrounded by family members.

For Americans accustomed to hospital delivery, home deliv- ery can seem like a risky proposition. In fact, in the least devel- oped countries of the world, where hospital delivery is far less common, the neonatal mortality rate (number of infants who live less than a month) is nine times higher than in the United States. In India alone, nearly half a million babies die before they are a month old; many parents do not name their new-

borns so that they will not become attached to a child who is likely to die (UNICEF, 2007).

The statistics are shocking, but you should not take them as an argument for the necessity of hospital births. In many of the least developed countries of the world, traditionally no trained health care professionals have been present at birth. When such professionals (typically a midwife) are present, labor and delivery become much safer for mother and infant alike, even when delivery takes place at home. Of course, sometimes problems emerge during pregnancy and labor; in these instances, ready access to a medical facility is essential. Combining these two elements—a health care professional present at every birth and specialized facilities available for problems— reduces neonatal mortality substantially (WHO, 2005).

This combination also works well in developed countries. Birth at home is safe if a woman is healthy, her pregnancy has been problem-free, the labor and delivery are expected to be problem-free, and a trained health care professional is there to assist (Olsen, 1997). Most women are more relaxed during labor in their homes and enjoy the greater control they have over labor and birth in a home delivery. But if there is any reason to believe that problems requiring medical assistance might occur, labor and delivery should take place in the hospital.

American women who are reluctant to give birth at home can turn to birth cen- ters. These are typically smaller clinics that are independent of a hospital. A woman, her coach, and other family members and friends are assigned a birthing room that is often decorated to look more homelike. A doctor or nurse-midwife assists in labor and delivery, which takes place entirely in the birthing room, where it can be observed by all. Like home deliveries, birthing centers are best for deliveries that should be trouble- free. Many hospitals now include such birthing centers, which allows women to be more relaxed and comfortable yet provides immediate access to specialized treatment should the need arise.

| Adjusting to Parenthood

For parents, the time immediately after a trouble-free birth is full of excitement, pride, and joy—the much-anticipated baby is fi nally here! But it is also a time of adjustments for parents. A woman experiences many physical changes after birth. Her breasts be- gin to produce milk and her uterus gradually becomes smaller, returning to its normal size in 5 or 6 weeks. And levels of female hormones (e.g., estrogen) drop.

Parents must also adjust psychologically. They reorganize old routines, particularly for fi rst-born children, to fi t the young baby’s sleep–wake cycle. In the process, fathers sometimes feel left out when mothers devote most of their attention to the baby.

Researchers once believed that an important part of parents’ adjustment involved forming an emotional bond with the infant. That is, the fi rst few days of life were thought to be a critical period for close physical contact between parents and babies; without such contact, parents and babies would fi nd it diffi cult to bond emotionally

In many countries around the world, a midwife

delivers the baby.

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Tu rn

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(Klaus & Kennell, 1976). Today, however, we know that such contact in the fi rst few days after birth—although benefi cial for babies and pleasurable for babies and par- ents alike—is not essential for normal development (Eyer, 1992).

Becoming a parent can be a huge adjustment, so it’s not surprising that roughly half of all new mothers fi nd that their initial excitement gives way to irritation, resent- ment, and crying spells—the so-called “baby blues.” These feelings usually last a week or two and probably refl ect both the stress of caring for a new baby and the physi- ological changes that take place as a woman’s body returns to a nonpregnant state (Brockington, 1996).

For 10 to 15% of new mothers, however, irritability continues for months and is often accompanied by feelings of low self-worth, disturbed sleep, poor appetite, and apathy—a condition known as postpartum depression. Postpartum depression does not strike randomly. Biology contributes: Particularly high levels of hormones during the later phases of pregnancy place women at risk for postpartum depression (Harris et al., 1994). Experience also contributes: Women are more likely to experience post- partum depression when they were depressed before pregnancy, are coping with other life stresses (e.g., death of a loved one or moving to a new residence), did not plan to become pregnant, and lack other adults (e.g., the father) to support their adjustment to motherhood (Brockington, 1996; Campbell et al., 1992).

Women who are lethargic and emotionless do not mother warmly and enthusi- astically. They don’t touch and cuddle their new babies much or talk to them. If the depression lasts only a few weeks, babies are unaff ected. However, if postpartum de- pression lasts for months, children of depressed mothers are more likely to become de- pressed themselves and are also at risk for other behavior problems (see, e.g., Dawson et al., 2003). In one study (Hay et al., 2003), when mothers had postpartum depression their children were more likely as 11-year-olds to be involved in aggressive behavior with peers (e.g., bullying them). One explanation of this fi nding emphasizes the role of early interactions with a mother in helping babies learn to regulate their emotions: When hungry, tired, uncomfortable, or frightened, infants with nondepressed moms soon learn that mom usually responds quickly and makes them feel better; over time, these infants become less upset when hungry or tired because they know that their discomfort will be brief. In contrast, when moms are depressed, they often fail to respond promptly to their infant’s needs, causing infants to become frustrated and angry with their lingering discomfort (Beebe et al., 2007).

Thus, postpartum depression is a serious condition that can harm moms and ba- bies alike; if a mom’s depression doesn’t lift after a few weeks, she should seek help. Home visits by trained health care professionals can be valuable. During these visits, these visitors show mom better ways to cope with the many changes that accompany her new baby. They also provide emotional support by being a caring, sensitive listener, and they can refer the mother to other resources in the community if needed. Finally, one simple way to reduce the risk of postpartum depression is worth mentioning— breast-feeding. Moms who breast-feed are less likely to become depressed, perhaps be- cause breast-feeding releases hormones that act as antidepressants (Gagliardi, 2005).

| Birth Complications

Women who are healthy when they become pregnant usually have a normal preg- nancy, labor, and delivery. When women are not healthy or don’t receive adequate prenatal care, problems can surface during labor and delivery. (Of course, even healthy women can have problems, but not as often.) The more common birth complications are listed in ● Table 2.6.

Some of these complications, such as a prolapsed umbilical cord, are dangerous because they can disrupt the fl ow of blood through the umbilical cord. If this fl ow of blood is disrupted then infants do not receive adequate oxygen, a condition known as hypoxia. Hypoxia sometimes occurs during labor and delivery because the umbilical cord is pinched or squeezed shut, cutting off the fl ow of blood. Hypoxia is serious be- cause it can lead to mental retardation or death (Hogan et al., 2006).

hypoxia

a birth complication in which umbilical

blood fl ow is disrupted and the infant

does not receive adequate oxygen

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To guard against hypoxia, fetal heart rate is monitored during labor, either by ultrasound or with a tiny electrode that is passed through the vagina and attached to the scalp of the fetus. An abrupt change in heart rate can be a sign that the fetus is not receiving enough oxygen. If the heart rate does change suddenly, a health care profes- sional will try to confi rm that the fetus is in distress, perhaps by measuring fetal heart rate with a stethoscope on the mother’s abdomen.

When a fetus is in distress or when the fetus is in an irregular position or is too large to pass through the birth canal, a physician may decide to remove it from the mother’s uterus surgically (Guillemin, 1993). In a cesarean section (or C-section) an incision is made in the abdomen to remove the baby from the uterus. A C-section is riskier for mothers than a vaginal delivery because of increased bleeding and greater danger of infection. A C-section poses little risk for babies, although they are often briefl y lethargic from the anesthesia that the mother receives before the operation. And mother–infant interactions are much the same for babies delivered vaginally or by planned or unplanned C-sections (Durik, Hyde, & Clark, 2000).

Birth complications are hazardous not just for a newborn’s health; they have long- term eff ects, too. When babies experience many birth complications, they are at risk for becoming aggressive or violent and for developing schizophrenia (Cannon et al., 2000; de Haan et al., 2006). This is particularly true for newborns with birth complications who later experience family adversity, such as living in poverty (Arseneault et al., 2002). These outcomes underscore the importance of excellent health care through pregnancy and labor and the need for a supportive environment throughout childhood.

Problems also arise when babies are born too early or too small. Normally, a baby spends about 38 weeks developing before being born. Babies born before the 36th

week are called preterm or premature. In the fi rst year or so, premature infants often lag behind full-term infants in many facets of development. However, by 2 or 3 years of age, such diff erences have vanished, and most premature infants develop normally (Greenberg & Crnic, 1988).

Prospects are usually not as bright for babies who are “small for date.” These infants are most often born to women who smoke or drink alcohol frequently during pregnancy or who do not eat enough nutritious food (Chomitz, Cheung, & Lieberman, 1995). Newborns who weigh 2,500 grams (5.5 pounds) or less are said to have low birth weight; newborns weighing less than 1,500 grams (3.3 pounds) are said to have very low birth weight; and those weighing less than 1,000 grams (2.2 pounds) are said to have extremely low birth weight.

Babies with very or extremely low birth weight do not fare well. Many do not survive, and those who live often lag behind in

the development of intellectual and motor skills (Sykes et al., 1997; Ventura et al., 1994). These impaired cognitive processes are shown in the Spotlight on Research feature.

● TA B L E 2 . 6

Common Birth Complications

Complication Features

Cephalopelvic disproportion When the infant’s head is larger than the pelvis, making it impossible for the baby to pass through the birth canal

Irregular position In shoulder presentation, the baby is lying crosswise in the uterus and the shoulder appears first; in breech presenta- tion, the buttocks appear first.

Preeclampsia A pregnant woman has high blood pressure, protein in her urine, and swelling in her extremities (due to fluid retention).

Prolapsed umbilical cord The umbilical cord precedes the baby through the birth canal and is squeezed shut, cutting off oxygen to the baby.

cesarean section (C-section)

surgical removal of infant from the uterus

through an incision made in the mother’s

abdomen

preterm (premature)

babies born before the 36th week after

conception

low birth weight

newborns who weigh less than 2,500

grams (5 pounds)

very low birth weight

newborns who weigh less than

1,500-grams (3 pounds)

extremely low birth weight

newborns who weigh less than 1,000

grams (2 pounds)

T H I N K A B O U T I T

A friend of yours has just given birth 6

weeks prematurely. The baby is average

size for a baby born prematurely and

seems to be faring well, but your friend

is concerned nonetheless. What could

you say to reassure your friend?

Small-for-date babies often survive, but their

cognitive and motor development usually is

delayed.

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K at

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Spotlight on Research Impaired Cognitive Functions in Low Birth Weight Babies

Who were the investiga-

tors, and what was the aim

of the study? Cognitive

development is often delayed in low birth weight

babies. Susan Rose and her colleagues (2005)

hoped to understand how impairments in basic

cognitive skills among low birth weight babies

contribute to delayed mental development dur-

ing the preschool years.

How did the investigators measure the topic of

interest? One of the earliest cognitive skills to

emerge is recognition memory, which refers to

the ability to detect that an object or event is

familiar—that it’s been experienced previously.

Rose and her colleagues measured visual recog-

nition memory by showing infants a photograph

of a face or a pattern for about 20 seconds. Then

this photograph was shown again, paired with

a novel face or pattern. Given the choice be-

tween novel and familiar stimuli, infants generally

look longer at the novel stimulus—assuming, of

course, that they recognize the familiar stimulus.

Consequently, Rose and colleagues recorded

percentage of time spent looking at the novel

stimulus as an index of recognition memory.

They also measured mental development

by administering the Mental Scale of the Bayley

Scales of Infant Development. This is a stan-

dardized test used for infants, toddlers, and

preschoolers that measure sensory, perceptual,

learning, memory, language, and problem-solving

skills.

Who were the children in the study? The sample

included 144 full-term babies who weighed at

least 2,500 grams at birth and 59 babies born

prematurely who weighed, on average, about

1100 grams at birth. The two groups of babies

were matched by gender (about even numbers of

boys and girls), by race (about 90% of the infants

were African American or Latino American), and

by mother’s education (an average of just over

13 years of education).

What was the design of the study? The study

was correlational because the investigators were

interested in the relation that existed naturally

between two variables: birth weight and cognitive

skill. The study was longitudinal because children

were tested twice: at 7 months of age they were

tested on the recognition memory task and at

3 years of age they were tested on the Bayley

Mental Scales.

Were there ethical concerns with the study? No.

The tasks were ones commonly used with infants

and preschool children; they posed no known

risks to children. The investigators obtained

permission from the parents for the children to

participate.

What were the results? The graph presented

as ❚ Figure 2.12 shows that, on the measure of recognition memory, the low birth weight babies

had lower scores. In other words, they did not

look as much at the novel stimuli, apparently

because they were less likely to recognize the

familiar stimulus as one they had seen previously.

The graph also shows that the low birth weight

babies had lower Mental Developmental scores.

The key result is a correlation of .44 between

recognition memory and mental development

scores: Infants who showed greater recognition

at 7 months had larger Mental Development

scores at 3 years.

What did the investigators conclude? Low birth

weight impairs basic cognitive processes—in

this case, the ability to recognize photographs

seen previously—and, over time, this impairment

leads to delayed development of a broad array of

mental skills.

What converging evidence would strengthen

these conclusions? The results show that low birth

weight affects children’s basic cognitive skills and

that this, in turn, leads to delayed mental devel-

opment. More convincing would be additional

longitudinal results showing that low birth weight

children with impaired basic skills are more likely

to be diagnosed with a learning disability, more

likely to repeat a grade, or less likely to graduate

from high school.

To enhance your understanding of this re-

search, go to www.cengage.com/psychology/

kail to complete critical thinking questions

and explore related websites. Data from

Rose et al. (2005).

Figure 2.12 ❚ Infants with low birth weight have less developed recog-

nition memory (they look less at novel stimuli) and, as

3-year-olds, have lower mental development scores.

Low birthweight

Full term

5550 60

Percent of time looking at the novel stimulus

Low birthweight

Full term

8580 90

Mental development score

The odds are better for newborns who weigh more than 1,500 grams. Most sur- vive, and their prospects are better if they receive appropriate care. Small-for-date babies are typically placed in special, sealed beds where temperature and air quality are regulated carefully. These beds eff ectively isolate infants, depriving them of envi- ronmental stimulation. You might think that stimulation is the last thing that these fragile creatures need, but sensory stimulation actually helps small-for-date babies to

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develop. Consequently, they often receive auditory stimulation, such as a tape record- ing of soothing music or their mother’s voice, or visual stimulation provided from a mobile placed over the bed. Infants also receive tactile stimulation—they are “mas- saged” several times daily. These forms of stimulation foster physical and cognitive development in small-for-date babies (Field et al., 2007; Teti, 2005).

This special care should continue when infants leave the hospital for home. Con- sequently, intervention programs for small-for-date babies typically include training programs designed for parents of infants and young children. In these programs, par- ents learn how to respond appropriately to their child’s behaviors. For example, they are taught the signs that a baby is in distress, overstimulated, or ready to interact. Parents also learn how to use games and activities to foster their child’s development. In addition, children are enrolled in high-quality child care centers where the curricu- lum is coordinated with parent training. This sensitive care promotes development in low birth weight babies; for example, sometimes they catch up to full-term infants in terms of cognitive development (Hill, Brooks-Gunn, & Waldfogel, 2003).

Long-term positive outcomes for these infants depend critically on providing a supportive and stimulating home environment. Unfortunately, not all at-risk babies have optimal experiences. Many receive inadequate medical care because their fami- lies live in poverty. Others experience stress and disorder in their family life. For these low birth weight babies, development is usually delayed and sometimes permanently diminished.

The importance of a supportive environment for low birth weight babies is under- scored by the results of a 30-year longitudinal study by Werner (1989, 1995) covering all children born on the Hawaiian island of Kauai in 1955. When low birth weight children grew up in stable homes—defi ned as having two mentally healthy parents throughout childhood—they were indistinguishable from children born without birth complications. However, when low birth weight children experienced an un- stable family environment—defi ned as including divorce, parental alcoholism, or parental mental illness—they lagged behind their peers in intellectual and social development.

Thus, when biological and sociocultural forces are both harmful—low birth weight plus inadequate medical care or family stress—the prognosis for babies is grim. The message to parents of low birth weight newborns is clear: Do not despair, because ex- cellent caregiving can compensate for all but the most severe birth problems (Werner, 1994; Werner & Smith, 1992).

| Infant Mortality

If you were the proud parent of a newborn and a citizen of Afghanistan, the odds are 1 in 6 that your baby would die before his or her fi rst birthday—worldwide, Afghani- stan has the highest infant mortality rate, defi ned as the percentage of infants who die before their fi rst birthday. In contrast, if you were a parent and a citizen of the Czech Republic, Iceland, Finland, or Japan, the odds are less than 1 in 300 that your baby would die within a year, because these countries have among the lowest infant mortal- ity rates.

The graph shown as ❚ Figure 2.13 puts these numbers in a broader, global con- text, depicting infant mortality rates for 15 developed nations as well as for 15 least developed countries. Not surprisingly, risks to infants are far greater—about 20 times, on average—in the least developed nations compared to developed nations (UNICEF, 2007). In fact, the diff erences are so great that the graphs for the two groups of nations must be drawn on diff erent scales.

If you’re an American, you may be surprised to see that the United States ranks near the bottom of the list of developed nations. The diff erence is small, but if the U.S. were to reduce its infant mortality rate to the 4% that’s common in European coun- tries, this would mean that 8,000 American babies who now die annually before their fi rst birthday would live.

infant mortality

the number of infants out of 1,000 births

who die before their fi rst birthday

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What explains these diff erences in infant mortality rates? For American infants, low birth weight is critical. The United States has more babies with low birth weight than virtually all other developed countries, and we’ve already seen that low birth weight places an infant at risk. Low birth weight can usually be prevented when a pregnant woman gets regular prenatal care, but many pregnant women in the United States receive inadequate or no prenatal care. Virtually all the countries that rank ahead of the United States provide complete prenatal care at little or no cost. Many of these countries also provide for paid leaves of absence for pregnant women (Kamer- man, 1993).

In least developed countries, inadequate prenatal care is common and mothers often have inadequate nutrition. After birth, infants in these countries face the twin

Figure 2.13 ❚ The infant mortality rate in least developed

countries is much higher than in developed

countries. Data from UNICEF 2007.Japan Sweden France

Germany Ireland

Israel Italy

Netherlands Spain

Australia Canada

New Zealand United Kingdom

United States Turkey

0 5 10 Infant mortality

(number of deaths per 1,000 births)

Developed nations

15 20 25

Source: Unicef 2007

Haiti Senegal

Sudan Bhutan

Cambodia Madagascar

Mauritania Uganda Somalia

Mozambique Rwanda Burundi Angola

Sierra Leone Afghanistan

60 80 100 Infant mortality

(number of deaths per 1,000 births)

Least developed nations

120 140 160

Source: Unicef 2007

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challenges of receiving adequate nutrition and avoiding disease. However, with im- proved prenatal care and improved health care and nutrition for infants, the global infant mortality has been cut in half since 1990 (UNICEF, 2007). With continued im- provements in such care, the main challenges for infants worldwide will be walking, talking, and bonding with parents—not sheer survival.

Recall answers: (1) placenta, (2) the pain-relieving medication crosses the placenta and

affects the baby, (3) when trained health care professionals are present to deliver the baby,

(4) hypoxia

Test Yourself

RECALL

1. In the third stage of labor, the

is delivered.

2. Two problems with using anesthesia during labor are that

a woman can’t use her abdominal muscles to help push the

baby down the birth canal and .

3. Home delivery is safe when a pregnant woman is healthy,

has had a problem-free pregnancy, expects to have a prob-

lem-free delivery, and .

4. When the supply of oxygen to the fetus is dis-

rupted because the umbilical cord is squeezed shut,

results.

INTERPRET

Explain why some at-risk newborns develop normally but oth-

ers do not.

APPLY

Lynn is pregnant with her fi rst child and would like to give

birth at home. Her husband is totally against the idea and

claims that it’s much too risky. What advice would you give

them?

2.1 In the Beginning: 23 Pairs of Chromosomes

What are chromosomes and genes? How do they carry heredi-

tary information from one generation to the next?

At conception, the 23 chromosomes in the sperm merge ■ with the 23 chromosomes in the egg. Each chromosome is one molecule of DNA; a section of DNA that provides specific biochemical instructions is called a gene.

All of a person’s genes make up a genotype; the pheno- ■ type refers to the physical, behavioral, and psychological characteristics that develop when the genotype is exposed to a specific environment.

Different forms of the same gene are called alleles. A ■ person who inherits the same allele on a pair of chro- mosomes is homozygous; in this case, the biochemical instructions on the allele are followed. A person who inherits different alleles is heterozygous; in this case, the instructions of the dominant allele are followed and those of the recessive allele ignored.

What are common problems involving chromosomes and what

are their consequences?

Most inherited disorders are carried by recessive alleles. ■ Examples include sickle-cell disease and phenylketonuria, in which toxins accumulate and cause mental retardation.

Sometimes fertilized eggs do not have 46 chromosomes. Usually they are aborted spontaneously soon after con- ception. An exception is Down syndrome, in which in- dividuals usually have an extra 21st chromosome. Down syndrome individuals have a distinctive appearance and are mentally retarded. Disorders of the sex chromosomes are more common because these chromosomes contain less genetic material than do autosomes.

How is children’s heredity influenced by the environment in

which they grow up?

Behavioral and psychological phenotypes that reflect ■ an underlying continuum (such as intelligence) often involve polygenic inheritance. In polygenic inheritance, the phenotype reflects the combined activity of many distinct genes. Polygenic inheritance has been examined traditionally by studying twins and adopted children and, more recently, by identifying DNA markers.

The impact of heredity on a child’s development depends ■ on the environment in which the genetic instructions are carried out, and these heredity–environment interactions occur throughout a child’s life. A child’s genotype can af- fect the kinds of experiences the child has; children and adolescents often actively seek environments related to

S U M M A RY

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their genetic makeup. Family environments affect sib- lings differently (nonshared environmental influence); parents provide a unique environment for each child in the family.

2.2 From Conception to Birth

What happens to a fertilized egg in the first two weeks after

conception?

The first period of prenatal development lasts 2 weeks. ■ It begins when the egg is fertilized by the sperm in the Fallopian tube and ends when the fertilized egg has im- planted itself in the wall of the uterus. By the end of this period, cells have begun to differentiate.

When do body structures and internal organs emerge in prena-

tal development?

The second period of prenatal development begins 2 ■ weeks after conception and ends 8 weeks after. This is a period of rapid growth in which most major body struc- tures are created. Growth in this period is cephalocaudal (the head develops first) and proximodistal (parts near the center of the body develop first).

When do body systems begin to function well enough to

support life?

The third period of prenatal development begins 9 weeks ■ after conception and lasts until birth. The highlights of this period are a remarkable increase in the size of the fetus and changes in body systems that are necessary for life. By 7 months, most body systems function well enough to support life.

2.3 Influences on Prenatal Development

How is prenatal development influenced by a pregnant wom-

an’s age, her nutrition, and the stress she experiences while

pregnant?

Parents’ age can affect prenatal development. Teenagers ■ often have problem pregnancies, mainly because they rarely receive adequate prenatal care. After age 35, pregnant women are more likely to have a miscarriage or to give birth to a child with mental retardation. Prenatal development can also be harmed if a pregnant mother has inadequate nutrition or experiences considerable stress.

How do diseases, drugs, and environmental hazards sometimes

affect prenatal development?

Teratogens are agents that can cause abnormal prenatal ■ development. Many drugs that adults take are teratogens. For most drugs, scientists have not established amounts that can be consumed safely.

Several diseases are teratogens. Only by avoiding these ■ diseases entirely can a pregnant woman escape their harmful consequences.

Environmental teratogens are particularly dangerous ■ because a pregnant woman may not know that these sub- stances are present in the environment.

What general principles affect the ways that prenatal develop-

ment can be harmed?

The impact of teratogens depends on the genotype of the ■ organism, the period of prenatal development when the organism is exposed to the teratogen, and the amount of exposure. Sometimes the effect of a teratogen is not evi- dent until later in life.

How can prenatal development be monitored? Can abnormal

prenatal development be corrected?

Many techniques are used to track the progress of pre- ■ natal development. A common component of prenatal care is ultrasound, which uses sound waves to generate a picture of the fetus. This picture can be used to determine the position of the fetus, its sex, and whether there are gross physical deformities.

When genetic disorders are suspected, amniocentesis and ■ chorionic villus sampling are used to determine the geno- type of the fetus.

Fetal medicine is a new field in which problems of pre- ■ natal development are corrected medically via surgery or genetic engineering.

2.4 Labor and Delivery

What are the different phases of labor and delivery?

Labor consists of three stages. In stage 1, the muscles of ■ the uterus contract. The contractions, which are weak at first and gradually become stronger, cause the cervix to enlarge. In stage 2, the baby moves through the birth ca- nal. In stage 3, the placenta is delivered.

What are “natural” ways of coping with the pain of childbirth?

Is childbirth at home safe?

Natural or prepared childbirth is based on the assumption ■ that parents should understand what takes place during pregnancy and birth. In natural childbirth, pain-relieving medications are avoided because this medication prevents women from pushing during labor and because it affects the fetus. Instead, women learn to cope with pain through relaxation, imagery, and the help of a supportive coach.

Most American babies are born in hospitals, but many ■ European babies are born at home. Home delivery is safe when the mother is healthy, when pregnancy and birth are trouble-free, and when a health care professional is present to deliver the baby.

What adjustments do parents face after a baby’s birth?

Following the birth of a child, a woman’s body undergoes ■ several changes: her breasts fill with milk, her uterus becomes smaller, and hormone levels drop. Both parents also adjust psychologically, and sometimes fathers feel left out. After giving birth, some women experience post- partum depression: they are irritable, have poor appetite and disturbed sleep, and are apathetic.

What are some complications that can occur during birth?

During labor and delivery, the flow of blood to the fetus ■ can be disrupted because the umbilical cord is squeezed

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shut. This causes hypoxia, a lack of oxygen to the fetus. Some babies are born prematurely and others are “small for date.” Premature babies develop more slowly at first but catch up by 2 or 3 years of age. Small-for-date babies often do not fare well, particularly if they weigh less than 1,500 grams at birth and if their environment is stressful.

What contributes to infant mortality in developed and least

developed countries?

Infant mortality is relatively high in many countries ■ around the world, primarily because of inadequate care before birth and disease and inadequate nutrition after birth.

chromosomes (42)

autosomes (43)

sex chromosomes (43)

deoxyribonucleic acid (DNA) (43)

gene (43)

genotype (44)

phenotype (44)

alleles (44)

homozygous (44)

heterozygous (44)

dominant (44)

recessive (44)

incomplete dominance (44)

sickle-cell trait (44)

phenylketonuria (PKU) (46)

Huntington’s disease (46)

behavioral genetics (47)

polygenic inheritance (48)

monozygotic twins (49)

dizygotic twins (49)

reaction range (50)

heritability coeffi cient (50)

niche-picking (51)

nonshared environmental infl uences (52)

prenatal development (53)

in vitro fertilization (55)

eugenics (55)

zygote (55)

implantation (55)

germ disc (55)

placenta (55)

embryo (56)

ectoderm (56)

mesoderm (56)

endoderm (56)

amnion (56)

amniotic fl uid (56)

umbilical cord (56)

cephalocaudal principle (56)

proximodistal principle (56)

period of the fetus (56)

cerebral cortex (57)

vernix (58)

age of viability (58)

spina bifi da (60)

stress (60)

teratogen (62)

fetal alcohol syndrome (62)

ultrasound (67)

amniocentesis (67)

chorionic villus sampling (68)

fetal medicine (68)

crowning (70)

doula (71)

hypoxia (73)

cesarean section (C-section) (74)

preterm (premature) (74)

low birth weight (74)

very low birth weight (74)

extremely low birth weight (74)

infant mortality (76)

K E Y T E R M S

Websites

Visit the Human Development companion website for all URLs.

The Human Development Book Companion Website ■

See www.cengage.com/psychology/kail for practice quiz questions, Internet exercises, glossary, fl ashcards, and more. Also accessible from the Wadsworth Psychology Study Center (www.cengage.com/login).

New York Online Access to Health (NOAH) ■

NOAH provides a wealth of information about all aspects of pregnancy and prenatal care.

Down Syndrome ■

The Down Syndrome website includes information about children who have this genetic disorder.

Human Genome Project ■

At the Human Genome Project website, you can see maps of each chromosome showing the location of known genes.

Go to www.cengage.com/login to link to CengageNOW, your online study tool. First take the Pre-Test for this chapter to get your Personalized Study Plan, which will identify topics you need to review and direct you to online resources. Then take the Post-Test to determine what concepts you have mas- tered and what you still need to work on.

L E A R N M O R E A B O U T I T

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Readings

DeSALLE, R., & YUDELL, M. (2004). Welcome to the ge- nome: A user’s guide to the genetic past, present, and future. New York: Wiley. The authors provide an excellent account of the history of genetics research and describe cutting-edge research on the human genome. They also talk about the ethical and social implications of greater understanding of the genome.

NILSSON, L., & HAMBERGER, L. (2003). A child is born (4th ed.). New York: Delacorte. This book is the source of many of the photos of prenatal development in this chapter. Nilsson developed a variety of techniques to photograph the

fetus as it was developing; Hamberger provides an entertain- ing and informative text to accompany the photos.

RIDLEY, M. (2000). Genome: The autobiography of a spe- cies in 23 chapters. New York: HarperCollins. The author describes progress in genetics research by telling fascinating stories about the impact of chromosomes on intelligence, language, cancer, and sex, to name just a few topics.

RUTTER, M. (2006). Genes and behavior: Nature–nurture interplay explained. Malden MA: Blackwell. This book, writ- ten by one of the leading researchers in the fi eld, provides a very readable introduction to research on the role of genetics in human behavior.

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