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Chapter Three

Prenatal Development

Conception

• Every 28 days an ovum is released

from one of a woman’s ovaries.

• Once released into the vaginal canal, sperm remain viable for up to 6 days.

• Conception takes place in the fallopian tube where sperm and ovum unite to form the zygote.

• Most conceptions result from intercourse on the day of or 2 days preceding ovulation.

The Period of the Zygote

• The period of the zygote lasts about 2 weeks, from fertilization until the cell mass drifts out of the fallopian tubes and attaches itself to the uterine wall.

• By the fourth day, 60 to 70 cells exist that form a hollow, fluid-filled ball called a blastocyst.

The Period of the Zygote Journey of the ovum to the uterus.

The Period of the Zygote

• Implantation

– Implantation occurs sometime between the seventh and ninth day when the blastocyst burrows deep into the lining of the uterus.

– The amnion is a membrane that encloses the developing organism in amniotic fluid. The fluid functions as a cushion and temperature regulator.

– The yolk sac produces blood cells until the liver, spleen, and bone marrow mature enough to take over this function.

– As many as 30 percent of zygotes do not make it through this phase.

The Period of the Zygote

– The placenta is a special organ that permits food and oxygen to reach the zygote and waste products to be carried away.

– The umbilical cord connects the placenta to the developing organism.

Period of the Embryo

• The period of the embryo lasts from implantation through the eighth week of pregnancy.

• The most rapid prenatal changes take place during these six weeks as the groundwork for all body structures and internal organs is begun.

Period of the Embryo • Last Half of the First Month

– During the third week (the first week of the period of the

embryo), the development of three cell layers:

• ectoderm-becomes the nervous system and skin • mesoderm-from which will develop muscles, skeleton,

circulatory system, and other internal organs • endoderm-becomes the digestive system, lungs, urinary

tract, and glands

– The nervous system develops fastest in the beginning. The neural tube is a primitive spinal cord that forms when the ectoderm folds over.

Period of the Embryo

• The Second Month

– The rapid development of body parts and systems continues. The embryo’s posture becomes more upright.

– The embryo can move, and it responds to touch, especially in the mouth area and on the soles of the feet.

Period of the Fetus • The period of the fetus is the “growth and finishing”

phase that lasts until the end of pregnancy.

• The Third Month

– The organs, muscles, and nervous system start to become organized and connected.

– By the twelfth week, the external genitals are well- formed, and the sex of the fetus can be determined using ultrasound.

– Trimesters are the three equal time periods in the prenatal period, each of which lasts three months

Period of the Fetus

• The Second Trimester

– By the middle of the second trimester (which lasts from 13 to 24 weeks), the fetus has grown large enough that the mother can feel its movements.

– Vernix is a white cheeselike substance that covers the fetus

and protects its skin from becoming chapped in the amniotic fluid.

– Lanugo is a white downy hair that also covers the fetus and helps the vernix stick to the skin.

Period of the Fetus • The Third Trimester

– The age of viability, between 22 and 26 weeks, is the age at which the fetus can first survive if born early.

– The brain continues to make great strides during the last three months.

– The cerebral cortex enlarges and the fetus spends more time awake. The fetus is also more responsive to external stimulation

Period of the Fetus

– The fetus moves less often, because of reduced space and a greater ability to inhibit behavior.

– A layer of fat develops under the skin to assist with temperature regulation.

– In the last weeks, most fetuses move into an upside-down position. Growth slows and birth is about to take place.

PRENATAL ENVIRONMENTAL INFLUENCES

• A teratogen is any environmental agent that causes damage during the prenatal period.

• Larger doses of teratogens over longer time periods usually have more negative effects.

• The presence of several negative factors at once can worsen the impact of a single harmful agent.

Teratogens

• The effects of teratogens vary with the organism’s age at the time of exposure.

– A part of the body is in a sensitive period when it is

undergoing rapid development; it is especially vulnerable to its surroundings during that time.

– The embryonic period is the time when serious defects are most likely to occur, since the foundations for all body parts are being laid down.

Teratogens and Timing of Their Effects on Prenatal

Development

Teratogens

• Prescription and Nonprescription Drugs

– Thalidomide, a sedative used in the 1960’s, caused severe limb deformations in embryos when taken by mothers between the fourth to sixth week after conception.

– Diethylstilbestrol (DES) was widely prescribed between 1945 and 1970 to prevent miscarriages. Daughters of these mothers showed unusually high rates of cancer of the vagina, malformations of the uterus, and pregnancies resulting in prematurity, low birth weight, and miscarriage.

Teratogens

• Repeated use of aspirin is linked to low birth weight, infant death around the time of birth, poorer motor development, and lower intelligence scores in early childhood.

• Heavy caffeine intake is associated with low birth weight, prematurity, miscarriage, and newborn withdrawal symptoms, such as irritability and vomiting.

• The safest course of action is to cut down or avoid these drugs entirely.

Teratogens

• Illegal Drugs

– Babies born to users of cocaine, heroin, or methadone are at risk for prematurity, low birth weight, physical defects, breathing problems, and death around the time of birth. In addition, these infants are often born drug-addicted.

– Evidence suggests that prenatal exposure to cocaine has lasting difficulties. These include genital, urinary tract, kidney, and heart deformities, as well as brain hemorrhages and seizures.

– Babies born to mothers who smoke crack are worst off in terms of low birth weight and central nervous system damage

Teratogens

• Fathers may contribute to these negative effects as cocaine may attach itself to sperm and cause birth defects.

• It is difficult to isolate the precise impact of cocaine, because users often take several drugs and engage in other high-risk behaviors.

• Mixed findings regarding the links between marijuana use and low birth weight or prematurity have been documented.

Teratogens

• Tobacco

– Effects of smoking during pregnancy include low birth weight and increased chances of prematurity, impaired breathing during sleep, miscarriage, infant death, and cancer later in childhood.

– Some studies report that youngsters exposed prenatally to tobacco have shorter attention spans, poorer mental health scores, and more behavior problems in childhood and adolescence.

Teratogens

• Alcohol

– Fetal alcohol syndrome (FAS) is the set of defects that results when women consume large amounts of alcohol during most or all of pregnancy. Symptoms include mental retardation; impaired motor coordination, attention, memory and language; overactivity; slow physical growth; and facial abnormalities.

– Fetal alcohol effects (FAE) is the condition of children who display some, but not all, of the defects of FAS. Usually the mothers drank alcohol in smaller quantities during pregnancy.

Teratogens

• Radiation

– When mothers are exposed to radiation during pregnancy, harm can come to the embryo or fetus.

– As demonstrated by children born to pregnant women who survived the bombing of Hiroshima and Nagasaki and the nuclear power plant accident in Chernobyl, radiation leads to a higher incidence of miscarriage and babies born with underdeveloped brains, physical deformities, and slow physical growth.

Teratogens

• Environmental Pollution

– An astounding number of potentially dangerous chemicals are released into the environment in industrialized nations.

– Established teratogens include mercury, lead, and polychlorinated biphenyls (PCBs).

Teratogens

• Maternal Disease

– Certain diseases during pregnancy can cause miscarriage and birth defects.

– Viruses

• Rubella (3-day or German measles) can cause a wide variety of abnormalities, especially when it occurs during the embryonic period.

• Acquired immune deficiency syndrome (AIDS), a disease that destroys the immune system, is infecting increasing numbers of women. When they become pregnant, they pass it to the developing organism 20 to 30 percent of the time.

Teratogens

• Bacterial and Parasitic Diseases

– Toxoplasmosis is a parasitic disease caused by eating undercooked or raw meat or contact with the feces of infected cats.

– During the first trimester, it leads to eye and brain damage.

Other Maternal Factors

• Exercise

– In healthy, physically fit women, regular exercise is related to increased birth weight.

– Since the growing fetus places some strain on the back, abdominal, pelvic, and thigh muscles, exercises that strengthen these areas are particularly helpful.

– In most cases, a mother who has remained fit during the earlier months experiences fewer physical discomforts later in pregnancy.

Other Maternal Factors

• Nutrition

– A healthy diet helps ensure the health of mother and baby.

– Consequences of Prenatal Malnutrition

• Autopsies of malnourished babies who died at or shortly after birth reveal fewer brain cells, a lower brain weight, and abnormal brain organization.

• Prenatal malnutrition can damage the immune system and the structure of organs, including the pancreas, liver, and blood vessels.

Other Maternal Factors

• Emotional Stress

– Intense stress during pregnancy is associated with a higher miscarriage rate, premature birth, low birth weight, newborn irritability, respiratory illness, digestive disturbances, and certain physical defects.

– When a mother experiences fear and anxiety, blood supply increases to the brain, heart, and limbs resulting in decreased blood supply to the uterus. Stress hormones also cross the placenta.

– Risks are greatly reduced when mothers have supportive significant others whom they can turn to for emotional support.

Berk, Laura E., (2016). Infants, Children, and Adolescents. Boston, MA: Pearson. EIGHTH EDITION