Information About Teratogenesis

Throughout most of human history, people believed that congenital anomalies were due to witchcraft or experiences of the pregnant woman impressing themselves upon the fetus. Early twentieth-century scientific thinking held that congenital anomalies were always genetic, despite the recognition a century or more earlier in Britain of the effect of alcohol on infants born to drinking mothers. Scientists and clinicians envisioned the placenta as a perfect barrier-protecting the fetus entirely from the environment.

Today, approximately 45-50 percent of congenital anomalies are believed to be due to a variety of inherited or chromosomal factors. Environmental substances, including drugs, are thought to account for approximately 7-10 percent of congenital anomalies, with drugs considered responsible for 2-3 percent of these. Current knowledge cannot explain the remaining 40-50 percent of birth defects.

Because teratogenic effects are often specific to the species, animal studies do not always provide definitive evidence regarding human teratogenesis. For obvious ethical reasons, scientists cannot conduct experimental studies in which a drug is given to some pregnant women and not to others to assess its effect on the newborn. Most information about the safety of drugs or other substances in pregnancy is based on reports of women exposed accidentally or because they simply needed a particular medication-regardless of possible adverse effects. The babies of women exposed or medicated can be compared to babies of women not exposed or medicated-the control group. These studies do not allow for as valid or reliable conclusions as experimental studies do.

Based on this sometimes imperfect knowledge, the Food and Drug Administration has developed the following classifications of drugs in pregnancy:

Category A: Controlled studies in women fail to demonstrate a risk to the fetus; the possibility of fetal harm is remote.

Category B: Animal studies do not indicate a risk to the fetus and there are no controlled studies in humans, or animal studies show adverse effects but controlled studies in humans have not shown a risk to the fetus.

Category C: Animal studies have shown the drug to cause adverse effects but there are no controlled studies in humans, or No studies are available in animals or humans.

Category D: Definitive evidence of risk to the human fetus exists, but the benefit in certain situations (e.g., life threatening situations in which safer drugs are unavailable or ineffective) may justify the use of the drug in those situations.

Category X: Studies in animals or humans have demonstrated fetal abnormalities or there is evidence of fetal risk based on human experience or both, and the risk dearly outweighs the possible benefits.

Approximately one-third to two-thirds of all pregnant women take at least one medication during pregnancy. Some of these are taken through out the pregnancy, others taken briefly to treat a particular condition that arises during pregnancy. No drug can be considered completely safe in pregnancy. Yet, many diseases are worse for both mother and baby than are the drugs used to treat them.