|Human Physiology 7/e Vander/Sherman/Luciano|
Online Learning Center
Some Causes of Birth Defects
The idea that the placenta protects the embryo and fetus from harmful substances was tragically disproven between 1957 and 1961, when 10,000 children in Europe were born with flippers in place of limbs. Doctors soon identified a mild tranquilizer, thalidomide, which all of the mothers of deformed infants had taken early in pregnancy, during the time of limb formation. The United States was spared a thalidomide disaster because an astute government physician noted adverse effects of the drug on monkeys in experiments, and she halted testing.
At about the same time that the medical community was noting the severity of the thalidomide crisis, another teratogen, a virus, was sweeping through the United States. In the early 1960s, a rubella epidemic here caused 20,000 birth defects and 30,000 stillbirths.
A pregnant woman who has just one or two alcoholic drinks a day, or perhaps a large amount at a crucial time in prenatal development, risks fetal alcohol syndrome in her unborn child. Because of the effects of small amounts of alcohol at different stages of pregnancy are not yet well understood, and because each woman metabolizes alcohol slightly differently, it is best to avoid drinking alcohol entirely when pregnant, or when trying to become pregnant.
A child with fetal alcohol syndrome has a characteristic small head, misshapen eyes, and a flat face and nose. He or she grows slowly before and after birth. Intellect is impaired, ranging from minor learning disabilities to mental retardation.
Teens and young adults with fetal alcohol syndrome are short and have small heads. Many individuals remain at early grade school level. They often lack social and communication skills, such as understanding the consequences of actions, forming friendships, taking initiative, and interpreting social cues.
Problems in children of alcoholic mothers were noted by Aristotle more than 23 centuries ago. In the United States today, fetal alcohol syndrome is the third most common cause of mental retardation in newborns, and 1 to 3 in every 1,000 infants has the syndrome--more than 40,000 born each year.
Chemicals in cigarette smoke stress a fetus. Carbon monoxide crosses the placenta and plugs up the sites on the fetus's hemoglobin molecules that would normally bind oxygen. Other chemicals in smoke prevent nutrients from reaching the fetus. Studies comparing placentas of smokers and non-smokers show that smoke-exposed placentas lack important growth factors. The result of all of these assaults is poor growth before and after birth. Cigarette smoking during pregnancy is linked to spontaneous abortion, stillbirth, prematurity, and low birth weight.
Certain nutrients in large amounts, particularly vitamins, act in the body as drugs. The acne medication isotretinoin (Accutane) is a derivative of vitamin A that causes spontaneous abortions and defects of the heart, nervous system, and face. The tragic effects of this drug were noted exactly 9 months after dermatologists began prescribing it to young women in the early 1980s. Today, the drug package bears prominent warnings, and it is never prescribed to pregnant women. A vitamin A-based drug used to treat psoriasis, as well as excesses of vitamin A itself, also cause birth defects. This is because some forms of vitamin A are stored in body fat for up to 3 years after ingestion.
Mainutrition in a pregnant woman threatens the fetus. Obstetrical records of pregnant women before, during, and after World War II link inadequate nutrition early in pregnancy to an increase in the incidence of spontaneous abortion. The aborted fetuses had very little brain tissue. Poor nutrition later in pregnancy affects development of the placenta. The infant has a low birth weight and is at high risk for short stature, tooth decay, delayed sexual development, learning disabilities, and possibly mental retardation.
Some teratogens are encountered in the workplace. Increased rates of spontaneous abortion and birth defects have been noted among women who work with textile dyes, lead, certain photographic chemicals, semiconductor materials, mercury, and cadmium. We do not know much about the role of the male in environmentally caused birth defects. Men whose jobs expose them to sustained heat, such as smelter workers, glass manufacturers, and bakers, may produce sperm that can fertilize an oocyte and possibly lead to spontaneous abortion or a birth defect. A virus or a toxic chemical carried in semen may also cause a birth defect.
MHHE Home | About MHHE | Help Desk | Legal Policies and Info | Order Info | What's New | Get Involved