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3.1 The Mechanics of Heredity
The mechanics of heredity are complex, its marvels, astounding, and the journey to which it leads can go down many different paths. The marvels are best left to poets, but the mechanics can be simplified.
Conception
Imagine one of the tiniest of all human cells, the sperm, a fragile thing, using whip-like motions of its long tail to propel itself upstream. It, along with perhaps another 3 or 4 hundred million sperm that have been produced in a man's testes and released at ejaculation during sexual intercourse, has undertaken a staggeringly immense journey across the vast stretch of the uterus and up a long fallopian tube.
There, should this sperm reach its goal, it will encounter the largest cell in the human body, an ovum (plural, ova; also referred to as the egg cell). The ovum measures about 0.15 millimeters in diameter (perhaps half the size of the period at the end of this sentence). Interestingly, all of a woman's ova are present in her ovaries at birth—as many as a million of them. They are primitive and immature, and more than half of them atrophy before puberty (the beginning of sexual maturity). Of those that remain, about 400 will mature and be released between puberty and menopause (cessation of menstruation), usually one every 28 days—but occasionally two, as is the case for fraternal twins. In contrast, a typical man produces sperm at the rate of several billion a month and usually continues to produce them from puberty until death.
Only about one in a million sperm—hence 300 or 400—that began the journey actually reach the end. But even now the goal is not yet in hand because there is a high probability that there will be no ovum here to fertilize, and the odyssey will all have been for nothing. But if there is an ovum at the end of the journey, it will usually be found about half-way up one of the fallopian tubes (about half the sperm have unluckily gone up the wrong tube). Having reached the ovum, the sperm now butts up against it, its tail whipping wildly, seemingly almost frantic to penetrate its shell (Figure 3.1; see also Figure 4.2, Chapter 4). If it succeeds in entering the egg, no other sperm will be so lucky because the ovum will immediately release enzymes to harden its shell, dooming all other sperm that have survived the journey!
Within a short while, there is fusion of the matter of which the sperm and ovum are composed and, voilà, we have conception—the creation of a fertilized egg (zygote). The zygote now drifts back down the fallopian tube, carried by slow currents. As it does so, the genetic material within the cell duplicates and forms an identical copy. The original and the identical copy of genetic material ease over to opposite sides of the cell, and the cell pulls apart and reforms, creating two identical cells—a type of cell division called mitosis.
The process is repeated again so that there are now four identical cells, and mitosis begins to accelerate so that by the end of the first week, there may be several hundred identical cells. By the time we are born, there will be tens of trillions; when we die, there will be hundreds of trillions. And each of them will contain an identical copy of our entire genetic blueprint.
This production of duplicate cells via mitosis is a very important feature of cell division; it explains why every one of the cells in our bodies (except for sex cells) has exactly the same genetic makeup—and why the forensic investigator can nail the criminal with a single cell from a drop of saliva as easily as with a cell from her blood, her hair, her skin, or her tooth. And it explains, as well, why the paleontologist dreams, with some reason for optimism, of recreating a woolly mammoth from a microscopic bit of its 10,000-year-old frozen carcass.
Body and Sex Cells
We have two kinds of cells in our body: sex cells (gametes), which consist of ova in the female and sperm in the male, and body cells (somatic cells).
All normal somatic cells form through mitosis and therefore contain identical genetic material. This genetic material is found on rod-like structures called chromosomes, of which there are 23 pairs in every human cell. These contain genes, which are the units of heredity (Figure 3.2). However, as sex cells mature, the chromosomes split through meiosis rather than mitosis so that each sperm and ovum only has half of each pair of chromosomes—hence 23 single chromosomes rather than 23 pairs of chromosomes. Furthermore, when chromosome pairs in the parent cell divide to form gametes, they do so randomly and genetic material crosses over from one chromosome to another. As a result, the genes that make up chromosome pairs are only one of a mind-boggling number of different possible combinations. And because there are two parents involved, the total number of different individuals that can result from a single human mating is an almost meaningless number larger than 60 trillion.
So should we be amazed that we are so much like our parents and siblings? Not really. You see, in these over 60 trillion theoretically possible combinations there is a vast amount of redundant information, much of which is absolutely fundamental to our humanity. That nearly everyone has a single head, two eyes, an astonishingly developed brain, functional digits, and on and on, is due to the secrets of the genetic code.
Why You Are Female or Male
Of the 23 pairs of chromosomes in the fertilized egg, a single pair of sex chromosomes determines whether the offspring will be male or female. As shown in Figure 3.3, the mother produces only the larger sex chromosome (the X chromosome); the father produces sperm that might have either the X chromosome or a smaller sex chromosome (the Y chromosome). If the sperm that fertilizes the ovum contains an X chromosome (resulting in an XX pairing), the offspring will be a girl; if it contains a Y chromosome (resulting in an XY pairing), the result will be a boy. Because the mother produces only X chromosomes, it is the father's sperm that determines the sex of the offspring.
The ratio of males to females at birth is about 105 to 100. But males are more susceptible to various diseases and accidents so that by the age of 5, there are almost as many girls as boys. By age 75, women outnumber men by about 4 to 3; more than 4 times more women than men live to be 100 (U.S. Census Bureau, 2010 Census Summary File 1).
The Genetic Code
That you are male or female is a biological accident, a fortuitous happening determined by a single pair of tiny chromosomes. In that pair of chromosomes, together with the other 22 pairs of chromosomes you inherited, lies your genetic code—your own, absolutely unique genetic blueprint.
Your genetic blueprint is coded in the form of arrangements of deoxyribonucleic acid (DNA) molecules that are located on your chromosomes. Although each of these molecules consists of an arrangement of only four chemical bases, or nucleotides (adenine, guanine, thymine, and cytosine, abbreviated AGTC), they are nonetheless highly complex explain Rosenfeld, Ziff, and Van Loon (2010). These four chemical bases also occur only in the pairs T–A or C–G (and the reverse, A–T and G–C), but because they occur in long interwoven strands (the shape, shown in Figure 3.4, is called a double helix), the number of possible variations is almost limitless.
Our genetic instructions, the most basic units of our genetic blueprints, are segments of these DNA molecules, called genes. In total, our chromosomes contain between 20,000 and 25,000 genes, which is far fewer than had been thought prior to the complete mapping of all the genes in human chromosomes, termed the genome (U.S. Department of Energy Genome Programs, 2008). These genes, either in pairs or in complex combinations of pairs, determine our potential for inherited characteristics.
How Genes Interact
The goal of genetics, simply put, is to identify and understand how genes function and interact. Especially relevant to psychology is behavioral genetics, which looks at the relationship between genes and human behavior.
Mendelian and Molecular Genetics
Mapping the human genome requires complex, highly sophisticated techniques, electron microscopes, and enormous computer power. These are the techniques of molecular genetics—techniques that make it possible to examine chromosomes directly, to look at sequences of DNA molecules, to identify their specific chemical components, to locate chemical segments that correspond to genes, and even to modify and to duplicate genes. The implications of research in this area are enormous. Correcting genetic defects might be possible, as might the alleviation of disorders and diseases caused by cell malfunctions.
In contrast to the sophistication of modern molecular genetics, the first scientific studies of inheritance required only a magnifying glass (or even just a good pair of eyes) a generous allotment of intelligence and inquisitiveness, and the patience of a monk.
The monk was Gregor Mendel, a nineteenth-century Austrian who bred, counted, and sorted generations of peas. By noting which peas had wrinkles and which didn't, and by relating them to parent plants as well as to offspring, Mendel discovered some of the first, and perhaps most important, secrets of genes—secrets having to do with dominance and recessiveness. Not surprisingly, the study of genetic dominance and recessiveness by looking at family resemblance is referred to as Mendelian genetics.
Dominance, Recessiveness, and Polygenetic Determination
Recall that genes come in pairs. Each member of a corresponding pair is called an allele. One allele has been inherited from the mother; the other member, from the father. Recall, too, that during meiosis, when the gametes (mature sex cells) are being produced, parent cells divide and are sorted randomly. What this means is that although the mother received half her genes from her father and half from her mother, her offspring might receive a preponderance of genes from one grandparent rather than the other.
The functioning of genes is responsible for our physical characteristics. For example, there are combinations of genes that correspond to eye color, hair characteristics, and virtually every other physical characteristic of an individual. In addition, other combinations of genes appear to be related to personality characteristics such as intelligence, and a variety of disorders, diseases, and defects.
The secret that Mendel discovered from his studies of peas is that in some situations, one member of a pair of genes— that is, one allele—may be dominant over the other. In such situations, if an individual inherits different alleles from each parent (is heterozygous for that gene), the characteristics corresponding to the dominant form of the gene will appear in the individual.
Eye color, for example, is clearly determined by genetics. The probability that identical twins will have the same color of eyes is 0.98 (Posthuma et al., 2006). But fewer than half of all fraternal twins, who are no more alike genetically than any other pair of siblings, have identical eye colors.
For many years, researchers believed that the allele for brown eyes is dominant and the one for blue eyes, recessive. This would mean that an individual who inherited an allele for blue eyes from one parent and one for brown eyes from the other would have brown eyes. And a blue-eyed individual would need to be homozygous with respect to eye color—that is, would have to inherit the recessive genes for this trait from each parent).
It turns out that the inheritance of eye color is not quite so simple (University of Queensland, 2007). In fact, eye color, like most other human characteristics, is a function of many pairs of genes acting in combination (termed polygenetic determination). This is why, no matter the color of parents' eyes, the child might have green eyes, blue eyes, brown eyes, grey eyes, or even eyes of different colors. There are situations, however, where the relative dominance or recessiveness of genes directly affects a characteristic. Figure 3.5 illustrates one such characteristic.
To further complicate matters, the DNA of which genes consist occasionally undergoes mutations—changes that may be brought about through X-rays, chemicals such as mustard gas, some drugs, or other causes. Mutations often occur prior to birth, especially during the early stages of prenatal development.
Genotype and Phenotype
our genetic makeup is your genotype; your manifested characteristics are your phenotype. Some aspects of phenotype, such as color of eyes or hair, can be observed directly; others, such as intelligence, are not so obvious. On the other hand, genotype is hidden and must either be inferred from phenotype or determined through an examination of the matter of which genes are composed.
It is possible to make accurate inferences about genotype for characteristics determined by recessive genes, but not for those determined by dominant genes. Normally, for example, the gene for non-red hair is dominant over that for red hair (Harding et al., 2000). Therefore we can infer that individuals whose phenotype (manifested characteristics) includes red hair must have two recessive genes for red hair (genotype). On the other hand, we can't be certain about the genotype for dark-haired individuals because they might have either two dominant genes for darker hair or a dominant gene for dark hair and a recessive gene for red hair.
In reality, the effects of genotype on phenotype are not quite as simple as the hair-color illustration implies. As we saw, most human characteristics, including hair color, are usually the result of combinations of genes (polygenetic). Often, as well, they are additive, meaning that phenotype results from the combined effects of alleles. As a result, the effects of polygenetic determination are not either/or (for example, either red or non-red), but include a whole range of possibilities such as auburn, golden, black, mousy, etc.
Canalization and Reaction Range
For some characteristics, such as being able to speak two languages, a person's environment makes a great deal of difference; for others, such as eye color, it makes little difference. Characteristics that are less affected by environmental forces, are said to have a higher degree of canalization, or heritability. High heritability for a trait means that genotype contributes significantly to the variations that might be seen in a population. Those aspects of the child's journey related to highly canalized characteristics will be more predictable.
For characteristics that are not highly canalized, phenotype may be very different from what might have been predicted on the basis of genetic makeup alone. High canalization of traits ensures that there will be a high degree of similarity among individuals of a species.
Even for highly canalized characteristics, however, developmental trajectories and outcomes may be very different from the expected. That's because phenotype (manifested characteristics) seldom results only from genetic influences, but is usually influenced by environmental factors as well. In a sense, it's as though genetic influences (genotypes) make possible a range of different outcomes, some more probable than others. Gottesman (1974) labels this range of possibilities reaction range. Simply stated, the reaction range for a particular characteristic includes all the possible outcomes for that characteristic given variations in the nature and timing of environmental influences. For example, your genetic makeup at birth might have been such that your most probable adult height would be 6 feet. But, given exceptional nutrition—and maybe as a result of hanging upside down with weights dangling from your ears—you might conceivably have succeeded in growing to 6 feet 8 inches and become a sought-after apple picker. Or, given poor nutrition and a long-term bout of some childhood disease, you could have ended up somewhat shorter at 5 feet 6 inches—and contented yourself with picking raspberries.
What is important from a psychological point of view is not so much what the individual contribution of heredity and environment might be, but rather how they interact to determine children's journeys.
3.2 Studying Gene-Environment Interaction
Understanding the interaction of heredity and environment has proven a difficult and controversial undertaking, which has led researchers up blind alleys more than once.
Feral Children
But that certain alleys are dead ends, or that they lead us away from our goals, is not always immediately apparent. For example, stories of feral children (wild children, ostensibly abandoned by their parents and brought up by wolves or wild dogs or tigers), have sometimes been used as evidence that we develop human characteristics solely as a function of interaction with other humans. Singh and Zingg (1942) describe 30 such stories, including that of Amala and Kamala, two girls reportedly dug out of a wolf's den in Northern India. They had supposedly been abandoned as infants and raised by a she-wolf. When they were dragged from the den, claim Singh and Zingg (1942), they were as wild as any wolf. And when they had been tamed somewhat, they continued to scurry about on all fours, growling at people, refusing cooked food, gnawing on bones, and devouring raw meat. Eventually they languished and died.
Most reports of wild children, explains LaPointe (2005), claim that when found, they typically walk on all fours rather than upright, many are unable to learn to speak, and most prefer uncooked food. But, he cautions, alleged cases of "wild" children, sensational as they are, are a rich source of deliberate hoaxes and unintentional exaggeration. When examined closely, the evidence is not very convincing. In fact, Dennis (1951) claims he couldn't find a single documented case of a child actually having been raised by a wild animal. The evidence, he says, is based solely on anecdotal reports of children whose identity and length of isolation have been unknown and who have never actually been observed with their supposed "adoptive parents."
But the literature has also reported several stories of abandoned children that are more believable. One is the story of Genie Wiley, a little girl whose journey was a sad venture. Unlike Amala and Kamala, Genie was abandoned in civilization rather than in the wilds: At the age of 20 months, she was closeted in a small, upstairs bedroom which, except for brief periods when she was still a toddler, she would not leave for more than 10 years.
Curtiss (1977) describes how Genie's room contained only a small crib entirely covered with wire mesh, and an infant potty. Day after day after day, she was left alone in this room, naked except for a leather harness that kept her strapped to the potty for hours at a time. Sometimes they forgot her on the pot at night; other times, they stuffed her into a straitjacket-type of sleeping bag and threw her in the crib, completely imprisoned within the wire mesh. Her father or, more rarely, her brother, would occasionally come in and feed her—almost always either baby food, soupy cereal, or a soft-boiled egg. Her father permitted no other contact with her and allowed no one to speak to her. Whoever fed her simply stuffed food into her mouth as quickly as possible. If she spit some of it out, her face would be rubbed in it. And if she cried or whimpered or made some other noise, her father would come in and beat her with a stick. He never spoke to her, but often pretended he was a dog, barking and growling at her and sometimes scratching her with his fingernails. If he wanted to threaten her, he would stand outside her door and make vicious dog noises.
When Genie was 13 years old, following an especially violent fight with the father, the mother finally took her and left. Shortly after that, Genie was discovered, charges were filed against the parents, and Genie was admitted to a hospital. Genie's father committed suicide on the day he was to be brought to trial.
Although Genie made some progress, her language and social development remained far below normal. After a few months of therapy, she could give one-word answers for some questions, and she had learned to dress herself (Rymer, 1994). Her doctors predicted she would recover and lead a normal life, and she moved into the home of one of her teachers, Jean Butler, and later into the home of psychologist David Rigler, where she stayed for 4 years. In 1975 she was returned to the custody of her mother, who soon decided that she could not care for her. She was then transferred to a succession of six foster homes and eventually, after losing the ability to speak, to a home for challenged adults (see Rymer, 1994; James, 2008 for more complete details).
Family Studies
From a psychological point of view, stories like Genie's are important to the extent that they shed light on how heredity and environment interact to direct children's journeys. The argument is that whatever characteristics these children share with others who are brought up in more "normal" environments result from genetic influences. By the same token, whatever "human" characteristics they fail to develop are more sensitive to environmental forces.
Another approach to this issue is to look at differences and similarities among members of a family. Why? Simply because family members share genes. As Plomin (1989) points out, there is 100% genetic similarity between identical twins, approximately 50% similarity among siblings who share both parents, and somewhere around 25% similarity among siblings who share only one parent.
It is the high degree of genetic relatedness among family members that led Francis Galton, Charles Darwin's cousin, to conclude that intelligence is largely hereditary. He had noticed that most of England's outstanding scientists came from just a few families.
Unfortunately, Galton's conclusions weren't based on very good research. Even if his observations were entirely accurate, they don't prove his point because it could be argued that the reason these families produced outstanding scientists was simply that they provided their children with environments that led to the development of genius. Still, Galton was convinced of the heritability of intelligence, and he argued that parents should be selected for favorable genetic characteristics—a practice termed eugenics.
Studies of Twins
One of the most fruitful approaches to determining whether Galton was correct is to study twins. That's because monozygotic twins (or identical twins) result from the splitting of a single fertilized ovum (zygote), forming two genetically identical zygotes. In contrast, dizygotic twins (or fraternal twins) are the product of two different egg cells fertilized by two different sperm cells, which results in twins who are no more alike genetically than ordinary siblings.
Frequency and Causes of Multiple Births
Unfortunately for researchers, the incidence of twins is relatively low—approximately 1 in every 86 births worldwide. Furthermore, identical twins are much rarer than fraternal twins—about 1 in every 250 births worldwide. (D'Addato, 2007).
In the United States today, twin births occur in approximately 3.2 of every 100 live births, a rate that is 70% higher than it was in 1980. Similarly, incidence of birth of triplets has increased by more than 400% since then (Martin et al., 2010). This increase is attributed mainly to an increase in age of childbearing (older women are significantly more likely to produce twins (Figure 3.6)) and an increase in the use of fertility-enhancing therapies such as drugs and assisted fertilization.
Studies of twins provide an exceptionally fruitful way of determining the extent to which characteristics such as intelligence, aggressiveness, and personality disorders are inherited. If these characteristics are mainly inherited, identical (monozygotic) twins should be highly similar. In contrast, fraternal (dizygotic) twins should be no more alike than other siblings. But if environment is most important, fraternal and identical twins should be very similar when they are raised together, and different when raised apart.
Furthermore, when identical twins are raised together, both their environments and their genes would be very similar. But when they are raised apart, their genes remain identical but their environments differ.
The Minnesota Twin Studies
One of the most important, ongoing longitudinal studies of twins is the Minnesota Twin Family Study at the University of Minnesota. It drew its samples of more than 8,000 twin pairs from all the twins born in Minnesota between 1936 and 1955, and between 1961 and 1964, and also included members of their families. Among other things, it looked at measures of intelligence, personality, interests, mental and physical health, and physiological measurements. In addition, it included a sample of twins who were reared apart (Iacono & McGue, 2002).
Results of these studies indicate that the median (midpoint) correlation for intelligence test scores for identical twins is above +0.80, whereas that for fraternal twins is below +0.60 (Bouchard & McGue, 1981). This appears to be true even for very young children.
If members of identical and fraternal twin pairs have had similar environments, these correlations may be interpreted as evidence that measured intelligence is influenced by heredity. With decreasing genetic similarity there is a corresponding decrease in the correlation in intelligence scores; the correlation for cousins is less than that for twins. This, too, is evidence of the influence of heredity.
But these data also support the belief that the environment influences measured intelligence. Because most sets of identical twins have more similar environments than do cousins or siblings, the higher correlations between various intelligence measures for identical twins may be due at least in part to their more nearly identical environments.
Especially intriguing among the Minnesota twins were several pairs of twins who were raised apart. Among them were twin boys who were separated at the age of 4 weeks and adopted by two families, the Lewises and the Springers, who lived 45 miles apart. Both sets of adoptive parents named their adopted son Jim. Jim Lewis knew he had a twin, but thought little about it; Jim Springer and his adopted parents thought his twin had died.
At the age of 38, the "Jim twins" were reunited and were astonished to discover how similar they were. Both weighed 180 pounds and stood 6 feet tall. Each had married twice, first to a "Linda" and then to a "Betty." Both had sons they named James (James Allan in the one case, James Alan, in the other). Both chewed their fingernails, had migraines, drank the same brand of beer, smoked the same cigarettes, and had dogs named Toy (Leo & Taylor, 1987).
The similarities between the Jim twins are uncanny, but, by itself, this single case offers little data of value to science. However, investigations of many other pairs of twins raised apart does. These investigations typically report slightly lower correlations for intelligence test scores for identical twins reared apart rather than together. In the Minnesota studies, the median correlations for those raised together was above +0.85; for those reared apart, correlations were around +0.75 (Bouchard, Lykken, McGue, Segal, & Tellegen, 1990). That the correlations are so high underlines the importance of genetic contributions. That they are somewhat lower for twins reared apart suggests that environmental forces are also important.
It's also revealing that as identical twins age, their phenotypes (manifested characteristics) become less similar. That is, correlations drop slightly, as shown in Figure 3.7. This provides additional evidence of the importance of gene-environment interaction.
Studies of Adopted Children
Like studies of twins, studies of adopted children are an important source of information about interactions among genetic and environmental influences. When it's possible to obtain information about both biological and adoptive parents, and about natural and adopted children, studies of adopted children permit a wide range of comparisons—for example, among individuals who share genes only (adopted children and a biological parent), who share environment only (adopted and natural children), or who share both (siblings in the same family).
In one longitudinal study, investigators looked at intelligence test scores of 245 adopted children and their biological and adoptive parents, as well as 245 other parents and their biological children. All the children were tested at various times between ages 1 and 16 (Plomin, Fulker, Corley, & DeFries, 1997).
So, how do we now evaluate the nature-nurture question? The issue is still not a simple one. And although the question has been posed since the beginnings of psychology, the answers that psychologists accept have changed dramatically over the years. For a long time, psychology believed that the reason some children are more intelligent than others is that they have inherited more "intelligence" from their parents. But by the 1970s, many came to believe that intelligence is determined mainly by a child's experiences, and there was a surge of emphasis on stimulating children, especially early in life. Now psychology is convinced that both heredity and environment are important, but not as isolated forces—rather, it is the interaction between the two that is important.
Gene-Environment Interaction
To understand the conclusion that developmental outcomes are the product of interactions among our genes and our environments, we have to understand what is meant by interaction. Interaction is not a simple concept; nor are its outcomes always highly predictable. Consider, for example, something as simple as the way changes in temperature transform water into ice. It seems a simple interaction between water and temperature. But the effect of temperature on water is not always ice: It can also be steam. That, too, appears to be a relatively simple, highly predictable interaction: More heat eventually equals steam; less heat eventually equals ice. But the interaction is not quite so simple because there are other variables involved, other interactions at play. For example, with changes in air pressure, the interaction of water and temperature changes so that now, more or less heat is required for the same effect.
So, too, with genes, environment, and human behavior; interaction is not a simple additive affair. Relative contributions of heredity and environment may change with a person's age, may be different in different environments, and may vary from one individual to another. Besides, children don't just inherit their parents' genes; they also inherit the parents themselves and perhaps an assortment of siblings and relatives, the places in which they live, and on and on. In Bronfenbrenner's terms, they inherit a whole entourage of systems with which they interact, and which shape their developmental outcomes.
Genes-Environment Correlation: The Scarr-McCartney Model
"The dichotomy of nature and nurture has always been a bad one," write Scarr and McCartney (1983). Why? Not simply because both influence the outcomes of development, but because they can also influence each other. For example, what we know of evolution makes it clear that gene combinations that underlie adaptive behaviors are more likely to survive in our gene pools; those that are maladaptive have a poorer chance of surviving. Thus does the environment influence genes.
It is perhaps not so obvious, but no less true, that genes can also influence environment. That's at least partly because we select and shape our environments in accordance with our genetically influenced abilities and interests.
Scarr and McCartney (1983) summarize many of these concepts in a model that attempts to explain the observation that genes and environments are often very closely correlated.
Passive Correlation
To begin with, say Scarr and McCartney, there is a sort of passive correlation between children's genes and the environment. This correlation results from the fact that parental characteristics that are themselves genetic influence the sorts of environments the parents provide for their children. For example, Sandra's highly intelligent parents might provide more books for her, might enroll her in a variety of courses, might take her on safaris, and so on. This source of correlation between the child's genotype and the environment is termed passive because the child is not deliberately involved in affecting the environment.
Evocative Correlation
Second, children's characteristics that are genetically influenced might have a direct bearing on their environments by eliciting certain responses from others—a process termed evocative correlation. For example, Robert's teachers might go out of their way to provide stimulating and expensive experiences (hence a different environment) for him because he seems to be a little prodigy.
Active Correlation
As they develop, children deliberately look for environments that reflect genetically influenced interests and abilities. This gives rise to what Scarr and McCartney label active correlation—active because the child purposely selects and shapes aspects of the environment. Thus Martha, who is athletic, fast, and strong, looks for opportunities to play organized sports and to work physically; and Alfredo, who has inherited his mother's talents, surrounds himself with music, and constantly begs his parents to hire music teachers. Both Martha and Alfredo actively strive not only to select environments compatible with their genotypes, but also to shape those in which they find themselves. The active selection of environments is labeled niche-picking.
Unfortunately, we can't always pick our niches. Sometimes forces more powerful than those we control—such as poverty, war, or wild beasts—send us on unexpected and unintended journeys.
Human Plasticity: The Rubber Band Hypothesis
Most psychologists believe that traits such as intelligence are inherited to a significant degree. At the same time, virtually all recognize that human characteristics are profoundly influenced by the interaction of genes and changing contexts.
In spite of this, the old nurture-nature question—the question of whether certain traits are influenced mainly by heredity or by the environment—has not gone away. It's a controversy that stems from an extreme and often emotional belief that all of us are—or at least should be—equal. And if we are equal, then it cannot be that Frances has an assortment of genes highly likely to lead to charm, intelligence, and grace, whereas Francis starts his life with an assortment of genes that propel him down the road to mediocrity or schizophrenia.
Or can it? Science suggests that yes, our genes are different and yes, our journeys may be vastly different as a result: We don't all have an equal probability of reaching the same destination. But science also tells us that genes, by themselves, determine little. Rather, they underlie potential, making some outcomes more probable than others. Even in the face of highly probable outcomes, environmental forces can lead to surprising and wonderful things. Or to things less wonderful.
That is the essence of Stern's (1956) rubber band hypothesis (Figure 3.9). In this model, plasticity is likened to the elasticity of a rubber band and the outcome of development, to its final length. Some are born with short bands (limited inherited potential for intelligence), others with longer bands. Some environments stretch bands a lot; others hardly stretch them at all; and perhaps some even shrivel them. Long bands, of course, stretch more easily. (See In the Classroom: Backpacks and One Shoe for a potentially band-stretching lesson.)
Do highly demanding environments break bands? Do old bands become frayed and brittle? Do bands stretch more easily when new? Unfortunately, analogies are simply comparisons; they provide no answers for questions such as these.
3.3 Inherited Abnormalities
To continue with the rubber band analogy, not only are there long and short bands, but there appear to be some that are brittle—and others that are astonishingly elastic. In this section, we look at brittle bands, at those that have defects. But we consider only a very small fraction of the thousands of conditions that are now known to be linked to genes or sometimes to defective chromosomes.
Gene-Linked Disorders
In most cases, serious genetic disorders are linked to recessive rather than dominant alleles. Recall that an allele is one of two corresponding forms of a gene, and that these can be homozygous (the same form) or heterozygous (different forms). The reason most severe abnormalities are linked with recessive forms of a gene is simple: Any abnormality that is linked with a dominant allele will always be manifested in all individuals with that form of the gene, and will have relatively little chance of being passed on to offspring (especially if it leads to early death). A disorder that is fatal in childhood and that is linked with a dominant gene should disappear from the human gene pool after a single generation.
In contrast, abnormalities that are linked to the recessive forms of a gene will be manifested only in individuals who have not inherited the normal allele from at least one parent. Many individuals may be carriers of a single recessive gene for some abnormality without manifesting the abnormality. Their offspring will also not manifest the abnormality unless both parents carry the relevant recessive gene and each passes it on.
Huntington's Disease
One exception to the general rule that most serious or fatal disorders are linked to recessive alleles is Huntington's disease, one of the few examples of a genetic disorder that shows complete dominance. This means that carriers of the gene almost invariably manifest the disease (Quarrell, 2008). For most other genetically linked disorders, dominance is often incomplete so that the disorder is not always apparent or it manifests itself in different, or weaker, forms.
In spite of being linked with a completely dominant allele, Huntington's disease is still present in the human gene pool because it does not ordinarily develop until the age of 30 or 40, thus giving carriers an opportunity to pass the gene on to their offspring. When Huntington's does appear, it leads to rapid neurological deterioration and eventual death.
Using the techniques of molecular genetics, geneticists have succeeded in locating the gene for Huntington's disease, making it possible to predict quite accurately the probability that an individual will be affected. But because Huntington's disease is still incurable and fatal, and because of medical insurance issues, fewer than one-quarter of those who might be carriers of the gene agree to be screened (Norrgard, 2008).
Sickle-Cell Anemia
Sickle-cell anemia is a genetic disorder linked to a recessive gene. In the United States, approximately 10% percent of African Americans, and a much lower proportion of whites, carry one recessive gene for sickle-cell anemia (and the corresponding normal allele) (Pace, 2007). These individuals are said to have sickle-cell trait, which rarely causes health problems.
Another 0.25% of the African-American population is homozygous recessive (individuals carry two defective genes). These individuals usually suffer from sickle-cell anemia (also called sickle-cell disease). Effects of the defective gene are clearly apparent in abnormally shaped red blood cells (sickle-shaped, rather than circular), which multiply with lack of oxygen (Figure 3.10). Sickle-shaped cells tend to clot together and thus carry even less oxygen, thereby increasing in number and reducing oxygen still more. Individuals who are homozygous recessive for this gene may be severely ill as a result. Although the condition is incurable, effective treatment is available.
PKU
Phenylketonuria (PKU) is a genetic defect associated with the presence of two recessive alleles. In individuals suffering from this disease, the liver enzyme responsible for breaking down the amino acid phenylalanine into usable substances is absent or inactive. Infants who inherit two recessive genes for PKU appear normal at birth, but with the continued ingestion of phenylalanine, their nervous system deteriorates irreversibly, and they become increasingly mentally retarded. Fortunately, however, PKU is easily detected at birth, and its onset can be prevented by providing children with diets low in phenylalanine (Filiano, 2006). Tests for PKU are routine in all U.S. states.
Muscular Dystrophy (MD)
Muscular dystrophy (MD) is a degenerative muscular disorder of which there are various forms. Some forms have been linked to a recessive gene. It sometimes involves an inability to walk and may lead to death. Also, it is often associated with lower mental functioning. Some forms of MD can be detected in fetuses by genetic screening. In addition, parents can be tested to estimate the probability that their offspring will be affected (Udd, 2011).
Neural Tube Defects
Among the most common congenital malformations (present at birth) are neural tube defects, which occur at a rate of 1 for every 1,000 births. Neural tube defects may take the form of spina bifida, in which the spine remains open at the bottom, or of anencephaly, in which portions of the brain fail to develop. Fetuses with anencephaly usually die in the womb or shortly after birth.
Spina bifida, which means cleft spine, ranges from so mild that it causes no problems and typically goes undetected (termed spina bifida occulta—meaning hidden) to forms that are accompanied by partial or complete paralysis and sensory deficits in the lower part of the body.
Although genetically linked, the causes of these defects are multifactorial. For example, a deficiency of the common B vitamin, folic acid, in the mother is closely associated with increased risk of neural tube defects (Taruscio, Carbone, Granata, Baldi, & Mantovani, 2011). As a result, many physicians recommend that mothers take folic acid supplements prior to and after conception. Most prenatal vitamins contain folic acid (sources of folic acid include peanuts, organ meats, asparagus, turnips, spinach, beans, and romaine lettuce).
Neural tube defects generally develop very early in pregnancy (as early as the first week) and can be detected by means of an AFP test—a standard test conducted at about the thirteenth week of pregnancy to ascertain the level of alpha-fetoprotein in the mother's blood. If this substance is present in higher-than-normal concentrations, additional tests are performed. In some jurisdictions, AFP screening is routine or even mandatory (Wyszynski, 2006).
Autism
Studies of twins suggest that autism spectrum disorder (or autism) also has high heritability (Freitag, 2007). Often, if one of a pair of identical twins is autistic, so is the other. Even when the second twin is not autistic, learning or social disabilities are common (Folstein & Rosen-Sheidley, 2001).
Indications are that transmission does not involve a single gene or gene combination, but is more likely associated with mutations in genetic material. This would explain why both members of identical twin pairs are often affected whereas only one of many siblings may be affected. Some evidence suggests that mutations linked to autism may involve as many as all 23 pairs of chromosomes (Szatmari et al., 2007).
Autism spectrum disorder, apparent primarily in the infant's failure to develop normal social and communication skills, is described in Chapter 6.
Schizophrenia
Schizophrenia, one of the most severe of mental disorders, comes in a variety of forms, most of which are characterized by emotional, cognitive, and perceptual confusion typically leading to a breakdown of effective contact with others and with reality. It takes various forms and often includes delusions, hallucinations, incoherence, or physical agitation. First diagnosis most often occurs in young adulthood.
Like autism, schizophrenia appears to have a strong genetic basis, although early environment— especially stressful or highly traumatic experiences—as well as various drugs, injury, and diseases are also important contributing causes (Picchioni et al., 2010). When one member of a twin pair has been diagnosed with schizophrenia, the probability that the other will also be is between 30 and 50% (Slatkin, 2009). Similarly, if one or both parents have schizophrenia, their offspring are significantly more likely to be diagnosed with schizophrenia (Lichtenstein et al., 2009).
Sex-Linked Defects
Some defects are associated with genes found on the X chromosome—one of two sex chromosomes (recall that an XX pairing is female; an XY pairing is male). Most common among them are hemophilia ("bleeder's disease"), night blindness, hereditary baldness, and color blindness. These conditions are referred to as sex-linked defects.
Each of these defects is associated with recessive genes, and each is more often manifested among males than females. Why? Simply because females who inherit the defective gene on one of their two X chromosomes typically have the corresponding normal allele on the other chromosome. As a result, they are carriers of the defective allele but they do not manifest it. But males have only one X chromosome and the normal dominant allele that would counter the effect of the recessive gene for one of these disorders is never present on the Y chromosome. Accordingly, males manifest conditions such as hereditary baldness and fragile X syndrome far more often than do females. Yet the gene is passed on from mother to son—not from father to son—because it is the father who always passes on a Y chromosome to his sons, and the mother who always passes on the X chromosome, potentially with the defective gene.
Chromosomal Disorders
In addition to gene-linked defects and diseases, some genetic abnormalities are chromosomal disorders; they are caused by defects in chromosomes rather than by specific genes. These defects might involve the absence of a chromosome, the presence of an extra chromosome, or a compressed or fractured chromosome.
Down Syndrome
The most common chromosomal birth defect is Down syndrome, which affects approximately 1 out of every 1,000 live births (Newberger, 2000).
Most cases of Down syndrome appear to be due to failure of the 21st pair of chromosomes to separate during meiosis (nondisjunction). Hence the resulting gamete (sex cell) has an extra copy of the 21st chromosome. When this gamete is combined with the other gamete during fertilization, the zygote (fertilized egg) has an extra 21st chromosome (thus the alternative medical label, trisomy 21).
Because nondisjunction of the 21st chromosome usually occurs during meiosis of the ovum rather than of the sperm, Down syndrome is often associated with the mother rather than the father. Increased probability of producing a child with Down syndrome is closely linked with the age of the mother, with the incidence ranging from about 1 in 1500 for women in their early 20s to about 1 in 20 for women above 45 (Figure 3.11). Older fathers also have a greater chance of fathering children with Down syndrome (Warner, 2003).
Some children with Down syndrome have characteristic loose folds of skin over the corners of the eyes; others don't. Mental retardation is common among children with Down syndrome, and their language development is often retarded. Not all children are equally affected.
Because medical science knows precisely what the genetic cause of Down syndrome is, it is possible to detect its presence in the fetus before birth.
Fragile X Syndrome
One of the most common causes of inherited mental retardation, fragile X syndrome, is associated with an X chromosome that is abnormally compressed or even broken. Although the condition can occur in females, it is far more common among males and is one of the reasons there are more mentally retarded males than females in the general population.
There is a very wide range of intellectual performance and developmental level among individuals with fragile X syndrome. Severe restlessness and hyperactivity are also often characteristic of males with fragile X syndrome (Fernández Carvajal, 2011).
Unlike Down syndrome, in which mental retardation is typically apparent very early in life, fragile X individuals often manifest no symptoms of retardation until puberty, after which there is frequently a marked decline in intellectual functioning. Because of this, fragile X syndrome has historically been underdiagnosed, or diagnosed very late. Furthermore, its diagnosis remained very uncertain until the discovery of its genetic causes, coupled with recent advances in genetic testing.
Turner's Syndrome
Turner's syndrome is a chromosome disorder that affects about 1 out of 2,500 female children (Donaldson, Gault, Tan, & Dunger, 2006). These children are born with a completely or partially missing sex chromosome. The condition is given the shorthand notation 45,X (or 45,XO). (The number indicates the total number of chromosomes; the letters refer to the sex chromosomes present. Thus a normal male is denoted 46,XY and a normal female, 46,XX.)
Most fetuses with a missing X chromosome are aborted spontaneously; those that survive typically have underdeveloped secondary sexual characteristics, although this is not evident until puberty. Possible signs of the disorder include (1) swelling in the extremities that disappears with age, leaving loose folds of skin (webbing), particularly in the neck region, fingers, and toes, and (2) short stature. Mental ability is usually normal, although there are some indications of higher impulsivity, occasional attention problems, and sometimes learning problems (Donaldson et al., 2006). Injections of the female sex hormone estrogen before puberty are often helpful in bringing about greater sexual maturation, although Turner's syndrome females remain sterile.
Klinefelter's Syndrome
Klinefelter's syndrome is a chromosomal aberration that involves the presence of an extra X chromosome in a male child (hence 47,XXY). It is the most common sex chromosome disorder in males (Bojesen & Gravholt, 2011). It is marked by the presence of both male and female secondary sexual characteristics. Children suffering from this disorder typically have small, undeveloped testicles, more highly developed breasts than is common among boys, highpitched voices, and little or no facial hair after puberty. Therapy with the male sex hormone testosterone is often effective in improving the development of masculine characteristics and in increasing sex drive.
XYY Syndrome
XYY syndrome—sometimes called "supermale" syndrome—occurs in males with an extra Y chromosome (47,XYY). Most males with this condition are not aware of it because it presents no unusual symptoms, although affected males are typically taller than expected (Cohen & Shim, 2007). Some research has linked this syndrome with criminality, but the conclusion was based on studies of incarcerated males. Most studies indicate that XYY males are not more likely to engage in criminal behavior (Witkin et al., 1976).
Physical and Personality Characteristics
In addition to various disorders and diseases, we know that physical traits such as eye color, hair and facial characteristics, body type, and so on, are, to a large extent, genetically based. For psychologists, an important, related question concerns the extent to which personality might also have genetic underpinnings.
One of the major emphases in genetics is the search for specific genes that contribute significantly to a characteristic or a behavior. One of the first steps in this search is to identify a gene that might contribute to a characteristic. Once that gene has been linked to a marker at a known location on a chromosome, and once researchers know the DNA sequence of the gene in question, they can create a mutation of it that, in effect, blocks its functioning. These mutant genes, termed knock-outs, can be used to study the contributions of specific genes to a wide range of characteristics.
Among the characteristics most often studied using this knock-out approach (and other tools of molecular genetics) are the many personality characteristics for which some genetic contribution is suggested by studies of human twins and of laboratory animals. For example, many studies have found very high concordance (percentage of shared characteristics) for personality characteristics such as anxiety, depression, conservatism, introversion/extroversion, obesity, and schizophrenia, for identical twins.
Obesity
Not surprisingly, family and twin studies suggest that some forms of obesity have a strong genetic basis. But, cautions Wells (2011), genetic contributions to the risk of obesity may have been overestimated. It's clear that human obesity is also environmentally determined, that it has much to do with what, when, and how much is eaten along the journey (as well as with physical activity on the way).
Modern techniques of molecular genetics have been used extensively in investigations of obesity, where much of the research has been conducted with rats and mice. And although no single "fat gene" has been discovered, there are indications that some types of obesity are at least partly controlled by certain proteins and neurotransmitters (Reed et al., 2011). In addition, there is evidence that the heritability of obesity may often depend on the timing of exposure to environmental influences. For example, some childhood diseases or medications taken in childhood might interact with genetic predispositions, increasing or decreasing the subsequent likelihood of obesity (Herrera, Keildson, & Lindgren, 2011).
Alcoholism
There are at least three lines of evidence to suggest that the propensity toward alcoholism has a significant genetic basis. We know that children of alcoholics are about four times more likely to be alcoholics than are members of the general population; studies of twins have found very high concordance rates for alcoholism; and certain racial groups appear to be more prone to alcoholism (Ystrom, Reichborn-Kjennerud, Aggen, & Kendler, 2011).
Although environmental influences (social and cultural factors and family habits and values, for example) may account for some of the higher incidence of alcoholism for some people, a fourth source of evidence is even more convincing: There appear to be detectable, biologically based differences between alcoholic-prone and "normal" individuals. Differences have been found with respect to certain liver enzymes involved in metabolizing alcohol, and with respect to specific brain receptors that might be associated with responses to alcohol (Strat, Ramoz, Schumann, & Gorwood, 2008).
Some Cautions
News media provide our most common source of information about genetics. Unfortunately, says Conrad (1997), they tend to sensationalize and to exaggerate: "Obesity gene discovered!" "Gay gene identified!" "Scientists finds gene for schizophrenia!" Furthermore, they tend to ignore subsequent disconfirmations; these are seldom very exciting. As a result, discredited ideas often remain part of public knowledge long after science has abandoned them.
3.4 Genetic Counseling and Engineering
Recent advances in genetics have enormous implications for assessing genetic risk. For a disorder that is linked directly to the presence or absence of known genes or to known chromosomal abnormalities, it is often possible to identify affected children before birth, or to determine the probability that parents will produce children with the disorder. For disorders whose origins are only partly genetic or for which the genetic contributions are complex and not completely known, it is nevertheless often possible to determine the probability that a child will be affected.
Prenatal Diagnosis
Prenatal diagnosis, the assessment of aspects of the condition of the unborn, is an extremely important tool in assessing genetic risk. The main techniques for fetal diagnosis were once limited to amniocentesis, chorionic villus sampling (CVS), ultrasound, and radiography. Now, preimplantation diagnosis may also be available.
Amniocentesis
Amniocentesis, one of two invasive approaches to fetal diagnosis, involves inserting a hollow needle into the womb to extract a sample of the amniotic fluid surrounding the fetus. This fluid contains fetal cells, which can reveal the absence of chromosomes or the presence of extra chromosomes, the fetus's blood type, the chemical composition of the amniotic fluid, and the genetic characteristics of the fetal cells.
Because the procedure involves a slight risk of infection and leads to miscarriages in a small number of cases, it is commonly used only when the pregnant woman is older (usually above age 35) and there is therefore a higher probability of fetal complications such as trisomy 21 (Down syndrome). Amniocentesis is not usually performed until the fourteenth to twentieth week of pregnancy.
Chorionic Villus Sampling (CVS)
Chorionic villus sampling (CVS) (also called chorion biopsy) is another invasive medical procedure for obtaining and examining fetal cells. In CVS, a plastic tube is inserted through the vagina (or a fine needle through the abdomen) to obtain a sample of the chorion. The chorion is a precursor of the placenta and contains the same genetic information as the fetus.
The advantage of a chorion biopsy over amniocentesis is that it can be performed as early as seven weeks after conception. If the results lead to a decision to terminate the pregnancy, this can be accomplished more simply and more safely in the first trimester of pregnancy. Its disadvantage is that it carries a slightly higher risk to the fetus.
Ultrasound
Ultrasound (sometimes called sonogram) is one of several non-invasive prenatal diagnostic techniques. It uses sound waves to provide a computer-enhanced image of a fetus in real time and is among the least harmful and least traumatic of techniques currently available. New developments in ultrasound imaging allow the production of especially vivid, full-color, 3D images. In advanced clinics, practitioners might use telesonography—that is, ultrasound recordings transmitted to remote locations. This allows groups of physicians and technologists to view the same images simultaneously, and to consult with each other. Also, Doppler-sonography—which can detect movement of liquids—may be used for monitoring the physical development of the fetus and to detect the possibility of circulatory or cardiac problems.
Ultrasound imaging techniques provide the most exact means for estimating fetal age, detecting the position of the fetus, discerning changes in fetal position, detecting multiple pregnancies, and identifying a variety of growth disorders and malformations. Ultrasound is always used with amniocentesis or CVS to guide the physician, but is not a substitute for these procedures because it cannot provide samples of fetal cells.
Radiography
Some inherited disorders and malformations can be diagnosed through X-rays (radiography). However, because of the possibility that X-rays may harm the fetus, sonograms should be used instead whenever possible (Goldberg-Stein, Liu, Hahn, & Lee, 2011).
Preimplantation Diagnosis
All the fetal diagnostic techniques discussed so far can be carried out only after the fetus has begun to develop. However, it is now possible to determine the chromosomal structure of the fetus before the fertilized egg has implanted. One way of doing this is to remove a mature egg and fertilize it with the father's sperm (in vitro fertilization). The zygote is allowed to multiply to the eight-cell stage, and then a single cell is removed and examined for the presence of specific genetic disorders—all within a matter of hours. Removal of a single cell apparently does not affect subsequent development. And it is now possible to test every chromosome in the embryo for known defects. If none are found, the zygote can then be implanted in the uterus.
Preimplantation diagnosis brings with it the great advantage that it can identify problems before a therapeutic abortion would otherwise be necessary. However, its use is still limited to a handful of advanced, highly specialized medical centers. It is sometimes used for patients with unexplained recurrent miscarriages (Suzumori & Sugiura-Ogasawara, 2010).
Treating Genetic Disorders
Not all genetically based disorders are equally serious. Many can be treated and controlled; some can be cured. For example, the effects of PKU can be prevented through diet. And for many other conditions, it is sometimes possible to replace deficient enzymes, proteins, vitamins, or other substances (the use of insulin to control diabetes, for example). (See In the Classroom: Orange Juice for Colin.) Similarly, various drugs or even surgery may be used to control and sometimes cure genetic diseases (some forms of cancer, for example).
In addition to these medical treatments, there are several other possible treatments. One involves the use of clones (identical copies) of normal genes to replace those that are defective; a second is the use of modified genes that "knock out" a specific aspect of the functioning of a defective gene; another is the use of antibiotics or even of viruses to eradicate bacteria or other agents involved in the development of genetically linked diseases.
Genetic Counseling
Prenatal diagnosis provides a powerful tool for detecting potential problems prior to birth. For example, major birth defects, evident in structural problems in one or more parts of the body, can often be detected prenatally (Table 3.1) And even though there are some genetically based problems that cannot always be diagnosed with absolute certainty, in many cases it is possible to estimate their probability. In such cases, parents and physicians may be faced with difficult decisions involving serious ethical questions. In these situations, genetic counseling offers a needed service.
Genetic counseling is a branch of medicine and of psychology that attempts to provide counsel to physicians and parents. Such counseling typically strives to assess the probability of a defect's occurrence, its likely seriousness, the extent to which it can be treated or even reversed, and the best courses of action to follow once a diagnosis has been made and a decision reached. In many instances, genetic counseling takes place before conception and takes into account the age and health of the mother and the presence of genetic abnormalities in the parents' families. In other cases, genetic counseling occurs after conception.
Prenatal diagnosis and genetic counseling give rise to some serious ethical issues. Not everyone agrees that prenatal diagnosis should be used to identify children with Down syndrome or other conditions so that they might be aborted. This is partly why, in spite of the availability of genetic counseling in a number of medical, research, university, and community centers, it is not always widely used. Also, there is still a relatively widespread lack of knowledge about such counseling on the part of physicians and potential clients alike. In addition, psychological barriers such as fear, social stigma, religious values, and financial considerations can limit the use of genetic counseling and reduce its effectiveness even when it is available.
Prenatal diagnosis can also present some potential for abuse.
Genetics and the Future
Genetic counseling has traditionally been limited to advising prospective parents about the likelihood of their having a child with a given problem and providing them with information about their options. The options have typically involved either trying to conceive or having an abortion. It may be very different in the near future.
We appear to be on the threshold of a new age. Almost daily there are new discoveries in genetics. Scientists are now able to detect some genetic weaknesses and strengths from the very earliest moments of life. Our mushrooming knowledge of cell biology at the molecular level is opening new doors into the vast world of genetic engineering. Science is learning to decipher the codes that direct the structure and function of the protein molecules that are the fundamental units of our biological lives. It is now possible to alter genetic messages and change the possibilities implicit in our genes.
Genetic engineering now makes it possible to reproduce a sequence of genetic material, duplicate it, and introduce it into a plant, animal, or human genome. The results include new medications (such as bacteria-produced insulin), new refining and manufacturing processes (for example, engineered genes to produce proteins and enzymes), new foods (such as fungaland virus-resistant food crops), and even new lives (such as clones).
Genetic engineering opens up the possibility of separating X- and Y-bearing sperm, and inactivating one or the other to select the sex of children before conception. In the end, it might be possible to design a child that is exactly what the parents order. In fact, such a child might even be produced in an artificial womb.
There are important ethical issues involved here. Some concern the potential dangers of experiments that can produce new forms of life, the consequences of which are unimaginable. Others have to do with the morality of altering genetic codes, of creating or ending life, of surrogate mothering and artificial insemination, and of selling human eggs, sperm, embryos, or fetal tissue.
Perhaps by 2100 issues such as these will have become more important than the science that gave them birth, and another new discipline will have arisen to deal with them.
But we have not yet reached the year 2100, and we continue to struggle with questions whose answers are not at all clear. Although we know vastly more now than we did even 10 years ago, there is still much about the beginnings—and the ends—of children's journeys that we do not understand.
4.1 Conception and Pregnancy
My brother and I were wrong. Reproduction and birth are no longer always simple, clear-cut phenomena such as might be evident among horses with their particular brand of sense. Sometimes the very beginning of the child’s journey is assisted.
Assisted Reproduction Technologies
Assisted reproduction includes various approaches. It might involve artificial insemination, typically a clinical procedure wherein sperm from the father (sometimes anonymous) is introduced directly into the mother. It might also include the use of drugs to stimulate ovulation. Another possibility is to insert a donor ovum into a prospective mother. Assisted reproduction might also involve conception outside the mother’s body, either using a surrogate mother (where one woman bears a child for another) or in vitro fertilization (IVF) (literally meaning "in glass"). In IVF, sperm might unite with ova on their own, although in a laboratory; or the sperm might be artificially inserted into the ovum (intracytoplasmic sperm injection (ICSI) (Figure 4.1). The fertilized egg is typically allowed to go through several cell divisions, and the resulting embryo can then be implanted in the mother to develop as it normally would (embryo implantation—also termed zygote implantation). And prior to this, as we saw in Chapter 3, a single cell can be removed from the embryo for genetic analysis (preimplantation diagnosis).
Yet another possibility is that ova, sperm, or embryo (fertilized ova) can be frozen to be thawed and fertilized or implanted at a later date. Although rates of successful fertilization and/or implantation are lower with frozen than with fresh ova or embryos, more than half survive the freezing and thawing process (Scaravelli et al., 2010).
Detecting Pregnancy
No matter whether it’s assisted or natural, reproduction always begins with conception, the fertilization of an ovum (egg). As we saw in Chapter 3, when an ovum is fertilized by a sperm cell, the resulting zygote contains the individual’s entire genetic endowment in the arrangement of the DNA molecules that make up its 23 pairs of chromosomes. Normally, all changes that take place in this cell’s development will result from the interaction of genetic predispositions with the environment, both before and after birth.
Probable Signs of Pregnancy
Barring the use of chemical tests or a medical examination, there are very few absolutely certain signs of pregnancy before the later stages of prenatal development. Cessation of menses (menstruation) is one of the first probable indications. However, it wouldn’t normally be noticed until at least two weeks of pregnancy have passed (conception ordinarily occurs approximately two weeks after the last menstrual period). In addition, it may be caused by factors other than pregnancy, such as anorexia, malnutrition, overexercise, or certain medical conditions.
Morning sickness, another probable sign of pregnancy, does not affect all women, can easily be mistaken for some other ailment, and doesn’t usually begin until a month after conception. Changes in the breasts (enlargement, darkening of the aureoles, increased sensitivity) are also common, but are highly subjective and therefore quite unreliable.
Positive Signs of Pregnancy
Quickening, the movement of the fetus in the womb, is a more positive sign of pregnancy. But it is not usually noticed by the mother until the fourth or fifth month, and by then most women have realized for some time that they are pregnant.
Other positive signs include the fetal heartbeat, which can be heard with the aid of a stethoscope. Similarly, fetal movements can be detected by feeling the abdomen or sometimes simply by observing it. X-rays and ultrasound are two other methods of ascertaining the presence of a fetus.
The surest early means of detecting pregnancy involves chemical tests, now widely used both at home and by physicians. These tests detect changes in a pregnant woman’s urine or blood caused by increased production of the pregnancy hormone (human chorionic gonadotropin [HCG]). They provide results as early as two weeks after conception. Positive indications in early pregnancy tests are highly reliable. In fact, in controlled studies, some home pregnancy tests detect up to 97% of pregnancies on the day of the missed menstrual period (Cole, 2011). Negative readings are less accurate, especially early in pregnancy. Blood tests administered by physicians are highly reliable and also allow relatively accurate prediction of expected time of delivery
4.2 Stages of Prenatal Development
The gestation period (the time between conception and birth) varies considerably for different species: Cows take about as long as people; elephants need 645 days; dogs come to term in approximately 63 days; mules gestate for almost a year; but some hamsters get it all done in only 15 to 17 days (Gestation, 2011).
The human gestation period is 266 days from the time of conception—that is, 38 weeks. Physicians often divide this period into three trimesters:
Week 1 to week 12: First trimester
Week 13 to week 26: Second trimester
Week 27 to the end of pregnancy: Third trimester
Biologically, prenatal development is typically described in terms of three developmental stages with clear time boundaries: The germinal stage (first two weeks following conception); the embryonic stage (until the end of the eighth week); and the fetal stage (until birth).
The Germinal Period
Except in certain cases of assisted reproduction (such as in vitro fertilization), fertilization usually occurs in one of the mother’s two fallopian tubes, each of which links an ovary to the uterus (Figure 4.2). After fertilization, the zygote (fertilized ovum) is carried to the uterus by currents in the fallopian tubes, a process requiring between five and nine days. Cell divisions occur during this time. Nevertheless, the zygote is hardly larger at the end of the first week than it was at the time of fertilization, mainly because the cells of which it consists are considerably smaller than they originally were (Figure 4.3). This is not surprising because the ovum has received no nourishment from any source other than itself.
One week or so after fertilization, the ovum is ready to implant itself in the uterine wall (at this stage, many potential pregnancies fail to implant). To facilitate this process, the ovum secretes certain enzymes and produces tiny, tentacle-like growths, called villi, which reach into the lining of the uterus to obtain nutrients from blood vessels.
As implantation is occurring, the lining of the uterus engulfs the uterus and prepares to nourish it—which is why menstruation, which is the monthly shedding of the uterine lining, doesn’t occur after successful implantation. This is the beginning of the placenta—the organ that allows nutrients to pass to what later becomes the fetus and waste materials to be removed, while keeping the blood of the mother and of the fetus separate. In time, the umbilical cord connects the placenta and the fetus via the navel. The umbilical cord contains no nerve cells, so that there is no connection between the mother’s nervous system and that of the child in utero (in the uterus).
The Embryonic Period
The embryonic period begins at the end of the second week of pregnancy, following implantation of the zygote in the uterine wall. The normal course of physiological development in the embryonic and fetal stages is highly predictable and regular.
At the beginning of this stage, the embryo is still only a fraction of an inch long and weighs far less than an ounce. Despite its tiny size, not only has cell differentiation into future skin cells, nerves, bone, and other body tissue begun, but the rudiments of eyes, ears, and nose have also appeared. In addition, some of the internal organs are beginning to develop. In fact, by the end of the second week, a primitive heart is already beating. By the end of the sixth week, the embryo is between 1.5 and 2 inches long but still does not weigh an ounce. All the organs are now present, the whole mass has assumed the curled shape characteristic of the fetus, and the embryo is clearly recognizable as human. Arm and leg buds have appeared and begun to grow, resembling short, awkward paddles. External genitalia (sex organs) have also appeared.
The Fetal Period
By the end of the eighth week of pregnancy, which marks the beginning of the fetal period, the absolute mass of the fetus is still quite unimpressive to the untrained eye. At 10 weeks of pregnancy, it will still be less than 3 inches long and weigh only half an ounce. The head of the fetus is now one-third of its entire length; this will have changed to one-fourth by the end of the sixth month and to slightly less than that at birth.
During the third month of pregnancy, the fetus is sufficiently developed that if it is aborted it can make breathing movements and give evidence of both a primitive sucking reflex and the Babinski reflex (the infant’s tendency to fan its toes when tickled on the soles of its feet).
During the fourth month of pregnancy, the bones have begun to form, all organs are clearly differentiated, and there may even be evidence of movement. During the fifth month a downy covering, called lanugo, begins to grow over most of the fetus’s body. This covering is usually shed during the seventh month but is occasionally still present at birth.
Toward the end of the sixth month, it is possible to feel the fetus through the mother’s abdomen. The heartbeat is now much clearer and the eyelids have separated so that the fetus can open and close its eyes. If born now in a modern hospital, it would have a chance of surviving with sophisticated care to compensate for the immaturity of its digestive and respiratory systems.
The fetus’s growth in size and weight becomes more dramatic in the last few months of the final stage (Figure 4.4). Brain development is also particularly crucial during the last three months of pregnancy, as it will continue to be after birth, especially for the first two years of life.
Most of the physical changes that occur after the seventh month are quantitative (Figure 4.5). It is now largely a matter of accelerated physical growth: from 15 inches and 2.2 pounds in the seventh month, to 20 inches and 7.6 pounds at birth (38 weeks).
4.3 Drugs, Other Chemicals, and the Fetus
A wide range of influences can significantly affect the fetus’s developmental trajectory. Influences that cause malformations and physical defects in the fetus include teratogens and mutagens.
Teratogens (from the Greek word Teras, meaning monster) affect the embryo or fetus directly, and include various maternal illnesses, drugs, chemicals, and minerals. In contrast, mutagens cause changes in genetic material, which can then lead to malformations and defects, and which may therefore be passed on to offspring.
Prescription Drugs
Most pregnant women in North America take vitamins and other dietary supplements. And more than half take one or more prescription drugs (Kulaga, Zargarzadeh, & Berard, 2010). We know that many drugs can affect the fetus; but about many others, we are still uncertain. Because it is clearly unethical to use human subjects in controlled investigations of chemical substances whose effects may be injurious to the fetus, our information about the effects of drugs on the fetus is often based on studies of animals or on observations of human infants under poorly controlled conditions.
Among the better-known teratogenic prescription drugs are thalidomide (a sedative drug once prescribed mainly for morning sickness but now rarely used), which causes severe physical changes in the embryo; quinine (sometimes prescribed for malaria, lupus, and arthritis), which is associated with congenital deafness; barbiturates and other painkillers, which reduce the body’s oxygen supply, resulting in varying degrees of brain damage; and various anesthetics that appear to cross the placental barrier easily and rapidly and cause depression of fetal respiration and decreased responsiveness in the fetus.
Other suspected teratogens include lithium, which is used in the treatment of manic-depression, benzodiazapines (antianxiety drugs such as Valium and Librium), amphetamines (stimulants), and some antihistamines (Handal, Engeland, Ronning, Skurtveit, & Furu, 2011). When these are used therapeutically, their effects on the developing fetus may be very small. However, the effects of habitual use and of overdosing are less clear, with some research reporting increases in the risk of abnormalities.
Some evidence suggests that the drugs most often used for depression, SSRIs such as Prozac, may affect the fetus’s neurological development, resulting in increased prenatal activity (Mulder, Ververs, de Heus, & Visser, 2011). The longer term consequences of these drugs are unclear, although a review of the literature suggests that other than for paroxetine, SSRIs pose no significant risk of congenital defects (Soufia, Aoun, Gorsane, & Krebs, 2010). Much of the uncertainty stems from the fact that there are few studies that have compared the infants of depressed but untreated mothers with those of mothers who have taken SSRIs.
The prescription drugs mentioned in this section are only a few of the drugs that are known to be potentially harmful to the fetus. There are many others that do not seem to have any immediate negative effects, but whose long-term effects may be negative but are still unclear.
Over-the-Counter Medication
Among nonprescription drugs that may also have negative effects on the fetus is aspirin (acetylsalicylic acid), which may increase the tendency to bleed in both mother and fetus. Even relatively low doses of aspirin may be associated with a higher probability of intracranial hemorrhage in infants, especially those who are premature (Friedman and Polifka, 1996). In addition, some animal studies have found that aspirin may be associated with a higher probability of fetal malformations as well as subsequent behavioral problems (Khera, 2005).
Acetaminophen (Tylenol, Anacin 3, Datril, Tempra, and related products) appears to pose little risk to the unborn fetus in normal therapeutic doses. Over-the-counter antihistamines may have adverse effects and should be used with caution (Black & Hill, 2003).
There has been some question concerning the effects of megadoses of common vitamins. Evidence suggests that vitamin C (ascorbic acid), in usual therapeutic doses, poses little risk to the fetus. Similarly, normal doses of vitamin A (retinol) do not appear to have measurable negative effects, although very high doses might have (Ng, Ma, Leung, & Leung (2011). Vitamin B6 (pyridoxine), even in very high doses, is not associated with fetal problems. Finally, evidence suggests vitamin D deficiency may be associated with delayed bone calcification (hardening) in the fetus, as well as with gestational diabetes and other problems (Shin, Choi, Longtine, & Nelson, 2010). Severe overdoses of vitamin D may also result in physical and mental abnormalities in the fetus (Erkkola, Nwaru, & Viljakainen, 2011).
Although not exactly an over-the-counter drug, the effects of aspartame, the increasingly common sugar substitute, have been a source of considerable debate and continuing controversy. Though there are few reliable human studies, some animal studies indicate that aspartame may be associated with a somewhat higher risk of neurological problems that might not be evident until later in life (Portela, Azoubel, & Batigália, 2007).
Importance of Timing and Other Factors
At least five separate factors determine the potential effect of a drug (or other agent) on the fetus: the mother’s reaction to the drug; drug dosage and frequency of use; interactions with other drugs the mother is taking or might have taken in the past; the conditions for which the mother is taking the drugs; and the timing of drug use. These multiple factors make the task of determining likely effects highly complex. How these factors interact, and their importance, is often unclear.
What has been clearly established, however, is that the timing of exposure to possible teratogens is critical. Thus, exposure during the first two weeks after conception may be associated with implantation failure and lead to spontaneous abortion. Exposure during the embryonic stage (weeks 2 to 8) is generally associated with the most serious structural changes (physical deformities and abnormalities). This is because the embryonic period is marked by the most rapid development of organs. After this stage, the fetus’s basic structure has already been formed and is not as vulnerable to external influences.
Which period is most critical for different teratogenic agents depends a great deal on precisely what part of the fetus they affect. Thus, thalidomide led to missing limbs in the fetus only if the mother took it between the fifth and seventh weeks of pregnancy (Therapontos, Erskine, Gardner, Figg, & Vargesson, 2009).
Exposure to Chemicals
The developmental paths today’s children must take are littered with chemicals. There are so many of them in our environments—more than 100,000 are registered—that it is extremely difficult to separate their potential effects one from the other. Although the majority of these are "contained" in one way or another—that is, they do not find their way into our air, our water, or our food—and are essentially harmless, others are not so benign.
Many chemicals are known to be "endocrine disruptors" that can mimic naturally occurring hormones. As a result, they may have very subtle, long-term consequences that cannot easily be investigated or attributed to a specific causal agent. Some of these chemicals tend to accumulate in body fat, and their negative effects can be transmitted to the fetus either directly or through the mother’s milk. Other chemicals have more direct effects, sometimes causing serious fetal malformations or death.
One example of a highly toxic chemical is methylmercury, whose effects received worldwide attention following the births of a large number of severely deformed and retarded infants in Minimata Bay, Japan. The deformities were traced to the presence of high levels of mercury in the fish consumed in great quantities by inhabitants of this community (in this case, the mercury was an industrial waste). The effects of mercury are now known as Minimata disease.
Another teratogenic group of chemicals whose effects are well known is the polychlorinated biphenyls (PCBs) and related chemicals such as dioxin, often used in manufacturing herbicides or insecticides. They were banned in 1979.
Bisphenol A, used primarily to produce plastics and often found in food and drug containers, is a known endocrine disruptor. It appears to be associated with a higher incidence of miscarriage and with physical deformities (Kovacic, 2010).
In addition to the chemicals known to be harmful to the fetus, there are many toxic chemicals whose effects on fetal development are unknown, although we know how they affect children and adults. Lead is one example. It is present in some fuel emissions, in some paints (lead makes paints taste sweet, which might explain why children and animals sometimes eat paint chips or dust), in certain metal products, and elsewhere. It accumulates slowly in the body; and when it reaches sufficiently high concentrations, it can lead to serious physical and mental problems in children and adults (Ondeck & Focareta, 2009; EPA: United States Environmental Protection Agency, 2012).
Radiation
Radiation exposure, primarily through X-rays, is a well-known teratogen. The fetus is especially sensitive to radiation in earlier stages, when bones and organs are first being formed. Possible effects include growth retardation, deformities, abnormal brain function, and an increased risk of cancers in later life. However, small doses of radiation do not appear to pose a significant risk other than for a slightly higher probability of later cancers. After 18 weeks of gestation, only extremely high doses would normally affect the fetus (Radiation and pregnancy, 2011).
Caffeine, Alcohol, and Nicotine
In one large-scale survey, 29% of pregnant women in Australia drank alcohol while pregnant and 43% drank while breast-feeding (Maloney, Hutchinson, Burns, Mattick, & Black, 2011). In another study, 23% of young, African-American women reported smoking after they knew they were pregnant (Chung et al., 2010). Estimates are that slightly fewer than 20% of women in the United States smoke while pregnant (March of Dimes, Quick reference fact sheets: Smoking During Pregnancy, 2012).
Caffeine
The effects of caffeine on the fetus do not appear to be very dramatic. Earlier suggestions that there might be a somewhat higher probability of premature delivery and of lower birth weight among mothers who consume very high levels of caffeine were based mainly on animal studies. Studies with humans have found minor effects. A summary and evaluation of a large number of studies in this area concludes: "Moderate or even high amounts of beverages and foods containing caffeine do not increase the risks of congenital malformations, miscarriage or growth retardation (Brent, Christian, & Diener, 2011). However, as Mulder, Tegaldo, Bruschettini, and Visser (2010) report, caffeine does lead to increased fetal activity and heart rates.
Nicotine
The harmful effects of maternal smoking on the fetus have been well documented. These include a significantly higher probability of fetal death (stillbirth), often associated with placental problems in which the placenta becomes detached from the uterine wall. Maternal smoking is also a highly significant contributor to premature births and lower birth weight and to developmental problems that become evident later in life—for example, a higher risk of sudden infant death syndrome (SIDS), childhood cancers, impaired fertility, diabetes, and respiratory dysfunction (Bruin, Gerstein, & Holloway, 2010; Blood-Siegfried & Rende, 2010). There is also increasing evidence that exposure to second-hand smoke may be harmful to the fetuses of non-smoking pregnant women. A review of 19 studies concludes that pregnant non-smokers exposed to second-hand smoke are 23% more likely to experience stillbirth, and 13% more likely to give birth to an infant with congenital malformations (Leonardi-Bee, Britton, & Venn, 2011).
Alcohol and FAS Disorder
Alcohol consumption by pregnant women may be associated with premature birth and with various defects in their offspring labeled fetal alcohol spectrum disorder (FASD) (also referred to as fetal alcohol syndrome or FAS).
FASD involves abnormalities in three areas: retarded physical growth; central nervous system problems evident in neurological problems, developmental retardation, or intellectual impairment; and characteristic cranial and facial malformations. These malformations typically include a low forehead, widely spaced eyes, a short nose, a long upper lip, and absence of a marked infranasal depression (the typical depression in the center of the upper lip, extending upward toward the nose). FASD is an important cause of mental retardation (Riley, Infante, & Warren, 2011).
With humans it is difficult to conduct the types of experiments that would allow researchers to determine precisely the amounts of alcohol and the timing of intake that result in these effects. Nor is it possible to separate completely the effects of alcohol from those of other drugs that might accompany alcohol use, or from the effects of malnutrition, also a possible corollary of alcohol use. The current consensus is that even in small amounts alcohol may be harmful to the fetus, although it may not necessarily lead to full-blown fetal alcohol spectrum disorder (Ismail, Buckley, Budacki, Jabbar, & Gallicano, 2010).
Illegal Drugs
Many illegal recreational drugs—such as narcotics, marijuana, and cocaine—can affect a fetus adversely.
Narcotics
Babies born to mothers addicted to narcotics such as heroin are often themselves addicted and subsequently suffer clearly recognizable withdrawal, labeled the neonatal abstinence syndrome (NAS). Its symptoms may include tremors, restlessness, hyperactive reflexes, highpitched cries, vomiting, fevers, sweating, rapid respiration, seizures, abnormal heart rate, and sometimes death. NAS infants frequently need to be treated with substances such as methadone (Leeman, Brown, Albright, Skipper, His, & Rayburn, 2011). There is evidence, too, that when pregnant women are treated with methadone (or buprenorphine), as many as one-third of their infants later require treatment for NAS (O’Connor et al., 2011).
Cannabis
Cannabis (marijuana), the most widely used illicit drug worldwide, does not affect the fetus as dramatically as heroin, morphine, or cocaine, but its effects are clearly not benign. For example, there is mounting evidence that fetal exposure to cannabis might be associated with neurological changes putting individuals at risk for later emotional disorders (Jutras-Aswad, DiNieri, Harkany, & Hurd, 2009). There is also evidence of lower birth weights and a higher risk of prematurity for infants born to heavy marijuana users (El Marroun et al., 2009).Following an extensive review of a large number of studies in this area, Reece (2009) reports that cannabis use has been linked with many serious long-term psychiatric conditions, respiratory problems, heart disease, and a variety of cancers. The extent and severity of these possible outcomes depend on the frequency and timing of exposure.
Cocaine
Cocaine—and crack, which is easily manufactured from cocaine, is less expensive, and has far more intense and immediate effects—is another commonly abused illicit drug. Cocaine addiction is reportedly about five times more common than heroin addiction in the United States. Some reports claim it has reached epidemic proportions. However, estimates of illegal drug use are seldom very reliable (Figure 4.6).
Once thought to be relatively harmless, cocaine is now considered not only highly addictive, but also potentially very harmful to the unborn. Among potential negative outcomes, infants born to cocaine and crack users appear to have a higher probability of heart defects (Meyer & Zhang, 2009), are significantly smaller than average, and are more likely to suffer from neurological problems (Salisbury, Ponder, Padbury, & Lester, 2009), are at greater risk for adverse effects on brain structure and function (Bhide, 2009), and have a greater likelihood of manifesting behavioral and emotional problems as children (Lester & Padbury, 2009).
4.4 Maternal Characteristics and the Fetus
The large variety of drugs and chemicals to which the mother is exposed can have important consequences for the well-being of the fetus. So can a variety of other maternal characteristics such as her age, the quality of her nutrition, and her health.
Maternal Health
Many diseases and infections are known to affect the fetus. These include rubella (German measles), syphilis, and gonorrhea, each of which can cause mental deficiency, blindness, deafness, or miscarriages. Similarly, a thyroid malfunction in the mother or an iron deficiency in her diet may lead to subnormal mental development.
Uncontrolled diabetes, whether pre-existing or gestational diabetes, can also have serious consequences for the fetus. Diabetes is termed gestational when its onset occurs during pregnancy. It occurs in approximately 7% of pregnant women and usually corrects itself following delivery (American Diabetes Association, 2011). Prior to the discovery of insulin, fetal and even maternal deaths were not uncommon. Now, however, mortality rates among diabetic mothers are about the same as those among nondiabetic mothers. And with timely diagnosis and proper medical management, adverse fetal outcomes are generally low (Zisser et al., 2010). Management involves careful monitoring of mother and fetus to assess and control sugar levels (glycemic control). Simple self-monitoring procedures are available for in-home use.
Herpes simplex type 2 (genital herpes) is the most common of all sexually transmitted diseases. Although its effects on the mother are typically more annoying than debilitating, it can have serious effects on the fetus, particularly if the mother’s infection is active at the time of delivery. The probability that infants will contract the virus during birth is extremely high. Because the newborn does not possess many of the immunities that are common among older children and adults, the herpes virus may attack the infant’s internal organs, leading to respiratory, visual, cardiac, or nervous system problems, or even death (Massler et al., 2011). As a result, infants born to mothers infected with the herpes virus are often delivered through cesarean section to prevent infection, especially if the mother is suffering from an outbreak of herpes at the time of delivery.
Acquired immune deficiency syndrome (AIDS) is another sexually transmitted disease that is of considerable current concern. First reported in the United States in 1981, the disease remains incurable and ultimately fatal. Approximately 40,000 new cases of AIDS are reported in the United States each year (Centers for Disease Control and Prevention, 2010).
AIDS is caused by the human immunodeficiency virus (HIV) which is transmitted through the exchange of body fluids, primarily through blood/blood exchange or through semen/ blood exchange. Most infants and children who are infected acquire the virus directly from their mothers through blood exchange in the uterus or during birth. With the use of powerful new drugs during pregnancy (atazanavir, for example) the risk of transmission from infected mother to fetus has been reduced significantly (Fowler, Gable, Lampe, Etima, & Owor, 2010). However, the drugs cross the placental barrier easily and a therapeutic dose for the mother may be toxic for the fetus, sometimes causing liver damage (Tomi, Nishimura, & Nakashima, 2011).
Rh(D) Immunization
There is a particular quality of blood in Rhesus monkeys that is often, but not always, present in human blood. Because this factor was first discovered in Rhesus monkeys, it is called the Rh factor (short for Rhesus factor). Individuals who have this factor are Rh-positive; those who don’t are Rh-negative.
A specific component of the Rh blood group, labeled D, is especially important for the pregnant mother and her fetus. Introduction of Rh(D)-positive blood into an individual who is Rh(D)-negative leads to the formation of antibodies to counteract the D factor—a process termed Rh(D) immunization. If these antibodies are then introduced into an individual with Rh(D)-positive blood, they attack that person’s blood cells, causing a depletion of oxygen and, without medical intervention, death.
Unfortunately, this situation can occur in the fetus (termed fetal erythroblastosis) when the fetus has Rh(D)-positive blood and the mother is Rh(D)-negative. At one time, this condition was always fatal. Now, however, it is possible for a physician to monitor antibody levels in the mother’s blood, determining when levels are high enough to endanger the fetus. At this point, there are several alternatives (Elalfy, Elbarbary, & Abaza, 2011). If the fetus is sufficiently advanced, labor might be induced or a cesarean delivery performed, and the infant given a complete blood transfusion immediately. If the fetus is not sufficiently advanced, a blood transfusion may be performed in utero.
Fortunately, this type of medical intervention is not often necessary since the development of Rhogam (Rh Immune Globulin or RhlG), a drug that contains passive antibodies that prevent the formation of additional antibodies. When there is a risk of Rh(D) immunization, Rhogam is administered to the mother shortly after delivery
Maternal Emotions and Stress
A once-common folk belief was that the mother’s emotional states could be communicated directly to her unborn child. If the pregnant woman worried too much, her child would be born with a frown; if she were frightened by a rabbit, the result might be a child with a harelip; if she had a particularly traumatic experience, it would mark the infant, perhaps for life. Accordingly, she must try to be happy and have pleasant experiences so that the child could be born free of negative influences.
Most of these beliefs about pregnancy are simply tales. However, because of the close relationship between the mother and the fetus, a number of investigators have pursued the idea that stimuli affecting her will also have some effect on the fetus, however indirect. One theory is that, because of the close connection between mother and fetus, changes in the mother’s chemical balance accompanying intense emotions and stress might affect her unborn child. There is some evidence, for example, that infants of mothers who experience high stress during pregnancy tend to react more anxiously, be more irritable, cry more, and recover more slowly from stressful experiences (Davis, Glynn, Waffarn, & Sandman, 2011). There is evidence as well of slightly lower developmental scores among infants whose mothers had higher levels of cortisol during pregnancy (an indication of stress level) and whose psychological state indicated high stress (Davis & Sandman, 2010).
Maternal Age
Infants of both older and younger mothers are sometimes at a disadvantage.
Older Mothers
We know that the incidence of trisomy 21 (Down syndrome) is about 40 times higher for mothers over age 45 than for mothers aged 20 (Chapter 3). We know, as well, that the probability of fathering a child with Down syndrome is considerably higher for fathers over the age of 55. Fragile X syndrome is also more common with increasing maternal age, as are Klinefelter’s syndrome and trisomy 18 (associated with neural tube defects, congenital heart disease, growth retardation, and other problems).
However, perhaps in part due to the availability of prenatal diagnostic procedures that make it possible to determine the presence of a number of chromosomal abnormalities and other defects or diseases prenatally, increasing numbers of women (and men) are making the decision to have a family later. In fact, the greatest increase in fertility rates in recent years has been among women in their early 30s. About 14% of all current births in the United States are to women over age 35, compared with only 6% in 1980 (U.S. Census Bureau, 2010).
Teenage Mothers
Birthrates among teenage mothers remain very high relative to the rest of the population. In fact, in 2007, 10.5% of all births were to mothers under age 19 (U.S. Census Bureau, 2010).
Children born to younger teenage mothers are often at a physical, emotional, and intellectual disadvantage relative to children born to older mothers. There are more miscarriages, premature births, and stillbirths among teenage mothers, and surviving infants are more often the targets of abuse and neglect (Pattanapisalsak, 2011). Their developmental scores on various measures are often retarded, and incidence of low birth weight is about twice as high for teenage mothers.
Low birth weight and less advanced developmental scores may reflect a variety of factors other than age or maternal health, not all of which are always taken into account. Among these are the poverty and the lack of social, educational, and medical assistance that often accompany teen pregnancy. Persistent poverty is clearly associated with more negative developmental outcomes. In fact, children who live in poverty tend to do more poorly in school, have measurably lower IQ scores, and experience more social and emotional problems (Weck, Paulose, & Flaws, 2008).
In retrospect, it seems that it is not so much the age or the economic circumstances of the parent that are the important factors, but rather how these contribute to the health care available to the mother and their influence on parenting and home environment. All other things being equal and unless she is very young (below age 15), if a teenage mother and her infant receive the same medical attention as an older mother, the health and developmental status of her infant will be normal. Unfortunately, however, all other things are not often equal for the teenage mother.
Maternal Nutrition
Nutrition is one of the single most important external influences on the developing fetus.
Effects of Serious Malnutrition
During the German siege of Leningrad in World War II, many Russians lived near starvation in bitterly cold, unheated homes. Many died. But perhaps most striking, in one clinic that had seen an average of 3867 births a year for the preceding three years, only 493 infants were born in 1942 (Shanklin & Hoden, 1979). Most of these babies were born in the first half of the year and had been conceived before the famine was at its height; only 79 were born in the second half of the year.
Effects of Less Serious Malnutrition
Investigating the effects of less dramatic forms of malnutrition is difficult. Investigators can’t always separate the effects of malnutrition from the effects of other factors that often accompany malnutrition (poor sanitation, poor medical care, drug use, and so on). In addition, malnutrition is seldom limited to the prenatal period but usually continues into infancy and even childhood.
Nevertheless, studies with animals, as well as with humans, leave little doubt that malnutrition, especially protein and carbohydrate deprivation, has serious negative consequences for the developing brain and is often reflected in lower birth weight (Tzanetakou, Mikhailidis, & Perrea, 2011). We also know that deficiencies in folic acid are linked with the development of neural tube defects, and that iron and calcium are especially important for fetal development. Unfortunately, even women who are not undernourished may have inadequate stores of iron, folic acid, iodine, and calcium (Barger, 2010). For affluent, middle-class mothers, all of these are readily available in supplements as well as in natural food sources; sadly, for those who are less affluent, and perhaps less well educated, they may not be available.
Nutritional Requirements during Pregnancy
During pregnancy, the mother’s energy requirements and her metabolism change. The presence of a growing fetus means that the mother requires somewhere between 10 and 15% more calories. And metabolic changes include an increased synthesis of protein, which is important for the formation of the placenta and enlargement of the uterus; a reduction in carbohydrate consumption, the effect of which is to provide sufficient glucose for the fetus; and increased storage of fat to satisfy the mother’s energy requirements.
Not only must pregnant women increase protein intake, but there is also an increased need for important minerals (for example, calcium, magnesium, iron, and zinc) and vitamins (mainly B6, D, and E). Recommended dietary allowances for pregnant women range from 25 to 50% above those for nonpregnant women (Health Supplements Nutritional Guide, 2011).
Research indicates that average intake of these nutrients is often less than recommended for American women. Hence current medical advice emphasizes that what the woman eats is more important than how much. With respect to fetal brain growth, protein appears to be among the most important ingredients of a good diet.
Optimal Weight Gain in Pregnancy
Physicians now use the concept of body mass index (BMI), rather than weight alone, to determine if an individual is obese, overweight, underweight, or at a healthy weight. Body mass index reflects the relationship between height and weight. It is calculated by dividing your weight in kilograms by the square of your height in meters (see http://www.nhlbisupport.com/bmi/ for a quick BMI calculator). For adults, a BMI somewhere between 18.5 and 24.9 is considered ideal. Those above BMI 25 would be considered overweight; above 30 defines obesity.
Current medical advice contradicts the long-held belief that women should be careful to minimize weight gain during pregnancy. Infant mortality rates are often lower in countries where pregnant women gain significantly more weight. Maternal weight gain leads to higher fetal weight and reduces the risk of illness and infection. Accordingly, doctors who once cautioned women to limit their weight gain to about 10 pounds now suggest that the optimal weight gain for a woman who begins pregnancy at an average weight is between 25 and 35 pounds; it is even higher for women who are initially underweight. For women who are initially overweight or obese, recommended gains are correspondingly lower (15–25 and 11–20 pounds respectively) (Weight Gain During Pregnancy: Reexamining the Guidelines, 2009). Total recommended weight gain for women carrying twins is 35 to 45 pounds.
Unfortunately, these recommendations are probably most relevant for those least likely to be exposed to them and least able to comply with them. Malnutrition and starvation are seldom a deliberate choice.
4.5 Childbirth
Childbirth is something that happens more than 4 million times a year in the United States, although birth rates have dropped from 18.4 per 1,000 population in 1970 to 14.3 in 2007 (U.S. National Center for Health Statistics, 2010).
Birth in today’s industrialized nations is largely a medical procedure. Elsewhere people’s experiences of birth might be very different. It might occur in birthing huts, in fields, in forests, or wherever the mother happens to be. Such a birth was sometimes a solitary experience but often there were midwives, healers, or other attendants. In general, this more "natural" birth was believed to be simpler, shorter, and less painful than birth often is today. Goldsmith (1990) quotes a nineteenth-century traveler who had been trekking with the Guyana women of South America: "When on the march an Indian is taken with labor, she just steps aside, is delivered, wraps up the baby with the afterbirth and runs in haste after the others" (p. 22). (See Across Cultures: A !Kung Woman’s First Child.)
But we know too that in the past—and in underdeveloped societies—birth was often a tragic experience both for mother and infant. As George R. R. Martin (2011) put it when writing about life on the fictional island of Great Wyk: "Nine sons had been born from the loins of Quellon Greyjoy but only four had lived to manhood. That was the way of this cold world where men fished the sea and dug in the ground and died, whilst women brought forth short-lived children from beds of blood and pain" (p. 29).
Even in the United States only a century ago, more than 100 of every 1,000 infants died before the age of one. As recently as 1980, infant mortality (defined as death before age one) was 12.6 of every 1,000 infants. That number has now been reduced by almost 50% (Figure 4.8). In many of the world’s poorer countries, however, infant mortality rates are still above 100 per 1,000 (United Nations World Population Prospects: 2011,
Labor
Labor—the process by which the fetus, the placenta, and other membranes are separated from the mother’s body and expelled—usually begins gradually and proceeds through three stages. The process may begin naturally or may be induced by the physician. Induced labor typically requires breaking the amniotic sac (the sac filled with amniotic fluid in which the fetus develops) and injecting the mother with a synthetic form of the hormone oxytocin. The frequency of induced labor in the United States increased from 9.6% births in 1990 to 23.2% in 2007 (U.S. National Center for Health Statistics, "VitalStats," 2010).
Stage 1: Dilation and Effacement of the Cervix
The first stage of labor, dilation and effacement of the cervix, is the longest, lasting an average of 12 hours for a first birth and about half as long for subsequent births. It consists of contractions, involuntary spasm-like actions that exert a downward pressure on the fetus as well as a distending force on the cervix. Initially, contractions are infrequent and mild but gradually become stronger and last longer. Stage 1 culminates in sufficient dilation of the cervix (the opening to the uterus) to allow passage of the baby from the uterus.
Stage 2: Delivery
The second stage of birth, delivery, begins when the cervix has dilated to about 4 inches. In a normal delivery, it starts with the baby’s head emerging at the cervical opening and ends with the birth of the child (Figure 4.10). The second stage usually lasts no more than an hour and often ends in a few minutes.
The fetus ordinarily presents itself head first and can usually be born without the intervention of a physician. When the head of the fetus is too large for the opening provided by the mother, a physician may make a small incision in the vaginal outlet (an episiotomy), which is sutured after the baby is born. Complications can also arise from abnormal presentations of the fetus: breech birth (buttocks first), transverse (crosswise), or a variety of other possible positions. Some of these can be corrected before birth by turning the fetus manually in the uterus (version).
Birth may be accomplished without the use of pain medications, although they are frequently used to ease the pain of labor and delivery. Epidural blocks (a local analgesic, which can vary in strength) and spinal blocks (that effectively block the transmission of pain signals to the brain) are common choices. Also available are narcotics, tranquilizers, and other types of pain medication. The main advantage of epidural and spinal blocks is that they alleviate pain without significantly affecting labor or the fetus. Although narcotics and tranquilizers can be effective in decreasing pain perception, they might also temporarily depress the infant’s breathing and immediate reactivity following birth.
Toward the end of the delivery stage, the attending physician or nurse severs the neonate’s umbilical cord; places silver nitrate or antibiotic drops in its eyes to guard against gonococcal infection; and checks to see that its breathing, muscle tone, coloration, and reflexive activity are normal. Following this, the physician assists in the third and final stage of birth and evaluates the condition of the neonate (newborn).
Stage 3: The Afterbirth
In the third stage, the afterbirth—the placenta and other membranes—is expelled. This process usually takes less than five minutes and seldom more than 15.
When James White, as recently as 1880, tried to teach obstetrics to other doctors in the United States, he was ridiculed and driven into obscurity. What do these events indicate? (Based, in part, on Gillespie, 1998.)
The Mother’s Experience: Prepared Childbirth
The preceding discussion of labor is admittedly clinical and perhaps a little like the cold, antiseptic hospitals in which most North American babies are born: It says little about the magic and mystery of the process—nor about how mothers experience it.
The inexperienced mother sometimes approaches birth with some degree of apprehension; there is often pain associated with childbirth. However, advocates of prepared childbirth (also called natural childbirth) claim that through a regimen of prenatal exercises and adequate psychological preparation, many women experience relatively painless childbirths.
Natural childbirth, a phrase coined by British physician Grantly Dick-Read (1972), refers to the process of having a child without anesthetics. The Dick-Read process involves physical exercises, relaxation techniques, and psychological preparation for the arrival of the child, all directed toward a delivery in which painkillers are unnecessary. It has led to the development of a number of approaches to childbirth, including the use of hypnosis (a procedure called hypnobirthing, which typically involves self-hypnosis). Other methods of natural childbirth include the Lamaze method and the Bradley method, which teach expectant mothers breathing and relaxation exercises. and the Leboyer method, which advocates delivering the baby in a warm bath (Leboyer, 1975) to eliminate much of the shock of birth to the baby.
Hospitals or Homes, Doctors, Doulas, or Midwives?
Many mothers now have a choice about where and how to have their babies. And although many choose to have their babies by natural means, this doesn’t usually mean having them at home with assorted relatives, or in the corn field or potato patch. More often, it means that they will enroll in prenatal classes and learn a series of exercises and breathing skills. Most U.S. births still occur in hospitals, although length of hospitalization is considerably shorter than it once was (often only a matter of hours).
In some countries, such as Great Britain, many births are attended by midwives rather than physicians, a practice less common in North America. In 2008, for example, 28,357 U.S. births occurred at home and another 12,014 were in a non-hospital birthing center (U.S National Center for Health Statistics, 2010). What are termed nurse-midwives are allowed to deliver babies in clinical, hospital, or home settings, and are also able to prescribe medication when needed. However, with the medicalization of birth, the use of midwives has declined, even in Europe, and their role has changed. An increasing number of births are attended to by physicians, and when midwives are used, they are often part of a medical team.
In North America, a doula may also be used in childbirth. Doula is a Greek word for a woman who helps other women. Doulas are typically women, unrelated to the mother, who are hired (or, in some cases, who volunteer) to provide emotional and physical support during childbirth, as well as to provide information. Unlike midwives, doulas do not ordinarily perform any clinical tasks. Often, they also assist the mother after childbirth, helping her with breastfeeding, care of the newborn, and perhaps even an assortment of household tasks.
Postpartum Depression
As many as 10 to 15% of all women suffer from serious depression after giving birth (Halbreich & Karkun, 2006). It isn’t clear whether this postpartum depression is due to hormonal changes, to the effects of sedating drugs that might have been used in labor, to disruptions in lifestyle, or to other factors. Its predominant symptoms include sadness, hopelessness, loneliness, helplessness, and lack of energy. Symptoms typically appear after the first month and last for a significant period of time. Less severe is what is termed postpartum blues, a period of sadness and mild depression that lasts only a few days or a week and that affects as many as half of all new mothers (Depression during and after pregnancy fact sheet, 2009).
The Child’s Experience
How do children, whose journeys are the subject of this text, react to the process of birth? We can only guess. Still, it seems likely that they are largely indifferent to the process. After all, they cannot reason about it, they cannot compare it with other more or less pleasant states, they can do nothing deliberately to alter it, and they will not even remember it. But consider the incredibly dramatic difference that birth makes. Up to now, the child has been living in a completely supportive environment. Receiving nourishment, getting oxygen, eliminating wastes—everything has been accomplished without effort. The uterus has been kept at exactly the right temperature, the danger of bacterial infection has been relatively insignificant, and there have been no psychological threats—as far as we know.
Now, at birth, the child is suddenly exposed to new physiological and perhaps psychological dangers. Once mucus is cleared from its mouth and throat, the newborn must breathe unassisted for the first time. As soon as the umbilical cord ceases to pulsate, it is unceremoniously clipped an inch or two above the abdomen and tied off with a clamp. And the child is now completely separate—singularly dependent and helpless, to be sure, but no longer a physiological parasite on the mother.
But birth is not without danger for the newborn. Throughout the world, about one-third of all births are not attended by any trained health professional (Esch, 2011). These births are linked with significantly higher risk to both mother and infant (Risking death to give life, 2005).
Even in clinical settings, injuries, including brain damage, sometimes occur during birth, often resulting from the tremendous pressure exerted on the head—especially if labor is long and if the amniotic sac has been broken early, in which case the head, in a normal presentation, has been repeatedly pressed against the slowly dilating cervix. In addition, the infant must pass through an opening so small that deformation of the head often results. (For most infants the head usually assumes a more normal appearance within a few days.)
Perhaps the greatest danger of birth is from anoxia, a shortage of oxygen to the tissues, and especially to the brain. The term asphyxia is often used interchangeably with anoxia. Strictly speaking, asphyxia refers to unconsciousness resulting from too little oxygen and too much carbon dioxide in the blood.
The causes of anoxia can be fetal (for example, they might result from the umbilical cord being lodged between the fetus’s body and the birth canal, or being twisted and effectively blocked, disrupting the flow of oxygen through the cord—a situation referred to as prolapsed cord). They can also be maternal (for example, due to cardio-respiratory problems in the mother, resulting in inadequate blood flow via the placenta). Or they might be placental (for example, resulting from placenta previa, a condition in which the placenta detaches prematurely from the uterine wall, often necessitating an emergency delivery.
Anoxia may be evident in impaired neurological, psychological, and motor functioning associated with brain damage (sometimes including serious mental and physical problems evident in cerebral palsy, seizures, and even death).
Cesarean Delivery
In an increasing number of instances, medical intervention bypasses these three stages of birth through cesarean deliveries, which account for almost one-third of all births in the United States (Figure 4.11). In a cesarean delivery, birth is accomplished by making an incision through the mother’s abdomen and uterus and removing the baby. Cesareans are most often indicated when the mother’s labor fails to progress, if previous cesareans have been performed, when the fetus is in a breech presentation, or if the physician detects signs of fetal distress. Cesarean deliveries are ordinarily undertaken before the onset of labor, but they can also be performed after labor has begun.
Although cesarean deliveries have clearly saved the lives of many mothers and infants and alleviated much pain and suffering, the rapid increase in the proportion of cesarean births relative to nonsurgical births has been a source of concern (Ostovar et al., 2010). Although much of this increase clearly results from dramatic improvements in the physicians’ ability to monitor the fetus during labor, critics assert that not all cesarean deliveries are necessary (MacDorman, Declercq, & Zhang, 2010). When unnecessary, such surgery presents potential disadvantages and dangers not inherent in a routine delivery, including greater medical risk to the mother, a longer recovery period, and higher risk of infection. In addition, the use of anesthetics during surgery may depress neonatal responsiveness and may be related to the occasional respiratory problems of infants delivered by cesarean section (Simpson, 2010).
Preterm and Premature Infants
Infants born before completing 37 weeks of gestation are classified as preterm. Some of these may also be premature, but the expressions are not equivalent. Premature infants are those whose organs have not matured sufficiently to allow them to survive without medical intervention. In a sense, they have been put on life’s road too soon.
The vast majority of preterm babies are low birth weight (LBW) (weigh less than 2500 grams—about 5.5 pounds). At one time, premature infants almost invariably died, most of them from respiratory failure. But with modern medical procedures, an increasing number survive, some born after as little as 22 weeks of gestation (approximately 4 months premature) and weighing as little as 750 grams (1.65 pounds) or even less.
Preterm birth is one of the more serious possible complications of birth, affecting approximately 8% of all infants born in the United States (U.S. National Center for Health Statistics, 2009). Rates are higher worldwide and appear to be rising, and account for more than a quarter of all neonatal deaths (deaths within 28 days of birth). Preterm births are also closely linked with much higher rates of cerebral palsy, learning disabilities, respiratory problems, and sensory handicaps (Beck et al., 2010). The incidence and severity of these developmental problems is closely related to the extent to which the infant is preterm. A large-scale study of extremely preterm infants (born between the 22nd and the 25th week) found that at the age of 6, nearly half of these infants had significant learning and physical disabilities (Marlow, Wolke, Bracewell, & Samara, 2005).
Hack and colleagues (1992; Hack, 2009) have been following a group of 249 very low birth weight infants (under 1500 grams; about 3 pounds) through the first two decades of their lives. Comparisons of these children with controls of average birth weight reveal lower weight, height, and head circumference at age 8, significantly higher incidence of various illnesses and surgical procedures, and significantly poorer performance on most measures of intellectual functioning including intelligence, language, reading, mathematics, spelling, and motor abilities. Many experienced learning problems in kindergarten (Taylor, Klein et al., 2011).
Many of the disadvantages of very low birth weight persist into adolescence, when rates of chronic conditions continue to be higher (Hack et al., 2011). Probably because the brain development for these infants is incomplete at birth, they continue to manifest detectable brain abnormalities in adolescence (Taylo, Filipek et al., 2011).
A subgroup of these low birth weight infants who weighed less than 750 grams at birth (about 1.5 pounds) were at even greater disadvantage. Almost 25% had intellectual disabilities, about half were in special education classes, 25% had vision problems, and 25% suffered from measurable hearing loss.
It is worth noting that in spite of their higher probability of experiencing developmental and medical problems, in Hack’s words "the majority of [adult] preterm survivors born during the early years of neonatal intensive care do well and live fairly normal lives" (2009, p. 467). However, most of these adult survivors were born in the 1970s and early 1980s when survival rates were much lower. It is possible that those who survived then may be different from those who survive now.
Causes of Prematurity
The precise causes of premature delivery are unclear, although research has identified a number of factors that are related to its occurrence. These include poverty, malnutrition, mother’s age, smoking and other drug use, and the presence of various infections and diseases such as gonorrhea and chlamydia. In addition, infants from multiple births are more frequently premature than are infants from single births. In the United States, social class and race appear to be correlated with incidence of prematurity: Premature deliveries are about twice as common among black women. Worldwide, incidence of preterm births—not all of which are premature— is highest in the poorest countries (Figure 4.12).
There are no sure ways to prevent premature delivery, although various medications are sometimes effective in preventing or stopping premature labor (Premature labor, 2011). For women at greater risk, medical personnel might recommend bed rest, refraining from sexual intercourse, and avoiding the use of antibiotics and other drugs.
Care of Preterm Infants
Survival rates for preterm infants have increased dramatically in developed countries, where these infants are usually cared for in a neonatal intensive care unit (NICU). Prematurity is not inevitably linked with physical, psychological, or neurological inferiority. With adequate care, many premature infants fare as well as full-term infants.
Nutrition and Medical Care What are the dimensions of that care? First they include advances in medical knowledge and technology. Ventilators, for example, make possible the survival of infants whose heart and lungs are not sufficiently developed to work on their own. Cooling caps and blankets are sometimes required for infants who suffer from hypoxic ischemic encephalopathy, a disease of the central nervous system. And intravenous feeding is necessary for infants who are not sufficiently mature to eat and process nutrients on their own.
The contents of intravenous feeding are especially important because they substitute for nutrients that the infant would ordinarily have received as a fetus. Certain fatty acids appear to be crucially involved in brain growth and neuron development during the last trimester of pregnancy—roughly the period of intrauterine growth that a 27-week preterm infant would miss. Accordingly, premature infants’ nutrition must include these nutrients at the appropriate time and in the appropriate form. Their blood chemicals and minerals are monitored closely and their diets adjusted accordingly. Often, the mother’s breast milk may be pumped and sometimes fortified with minerals and vitamins, to nourish the infant. Not surprisingly, hospital care of premature infants is sometimes enormously expensive.
Psychological Care In addition to important medical advances in the care of premature infants, a tremendous amount of research in the past 20 years has investigated the possibility that at least some of the adverse psychological consequences of prematurity might be due to the lack of stimulation the preterm infant receives in an intensive care nursery. This has led to the development of what is termed kangaroo care, a procedure highly recommended for premature infants who do not require life support. It involves holding the infant against a parent or other caregiver’s bare chest, skin to skin, thus providing both warmth and comfort. Typically, the infant wears only a diaper and a hat, and is held against the parent by means of a wrap that ensures proper support as well as protection from temperature fluctuations. This provides the infant with tactile stimulation as well as with sounds of the caregiver’s voice and breathing, and perhaps a sensation of a heartbeat, all of which are important to the infant’s development.
Evidence suggests that kangaroo care results in the release of oxytocin, a hormone thought to be involved in the development of the brain and other systems (Ludington-Hoe, 2011). A number of studies suggest that kangaroo care is effective in reducing pain among both full-term and premature infants (Warnock et al., 2010).
The evidence seems clear that the traditional hands-off treatment once given most premature infants is not the best form of care for them.
Evaluation of the Newborn
When you leave on a long and important road trip, you have your vehicle inspected and any problems rectified. So, too, with the newborn who, at the end of this chapter, will finally be on the road.
In almost all North American hospitals it is routine to evaluate the condition of newborns by means of the Apgar scale. The scale, shown in Table 4.1, is almost self-explanatory. Infants receive scores (0, 1, or 2) according to the presence or absence of five indicators of well-being: appearance (color), pulse (heart rate), grimace (reflex irritability), activity (muscle tone), and respiration (note how the underlined letters are an acronym that spells Apgar). Maximum score of the combined indicators is 10; either 9 or 10 is considered "excellent," 7–8 is "good," 4–6 "intermediate," and below that, "poor." The Apgar evaluation is usually administered one minute and five minutes after birth (and sometimes 10 minutes after birth). Five- and ten-minute scores are often higher than one-minute scores. Almost 90% of U.S. infants score 9 or 10; only 0.5% score below 4 (Martin et al., 2010).
A second important scale for assessing the condition of a newborn infant is the Brazelton Neonatal Behavioral Assessment Scale (NBAS) (Brazelton, 1973; Brazelton Institute, 2011). Like the Apgar scale, it can be used to detect problems immediately after birth. In addition, it provides useful indicators of both central nervous system maturity and social behavior. The Brazelton scale evaluates the infant’s responses to 28 behavioral and 18 reflex items including reaction to light, to cuddling, to voices, and to a pinprick. It is particularly useful in identifying infants who might be prone to later psychological problems. (See In the Classroom: Preschoolers’ Health Problems.)
A Reassuring Note
It is often disturbing for nonmedical people to consult medical journals and textbooks in search of explanations for their various ailments; it’s easy to find that one has symptoms of some vicious infection or exotic disease. So if you happen to be pregnant at this moment or are contemplating pregnancy, you might find yourself a little apprehensive. Reassuringly, there is seldom cause for alarm. In many ways, the intrauterine world of the unborn infant is less threatening and less dangerous than our world. In most cases, when a fetus comes to term, the probability that the child will be normal and healthy far outweighs the likelihood that it will suffer any of the defects or abnormalities described in this chapter. Most children can look forward to a delightfully happy journey.