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Teratology is the study of the effects of teratogenic substances on the development of embryos, fetuses, or a pregnancy. Teratogenic substances can cause congenital malformations, or they can halt a pregnancy altogether. Clinical and experimental teratologists study how teratogenic agents that actively create birth defects can disrupt the normal development of a fetus or embryo. These include chemicals, drugs, maternal infections, and radiation. Exposure during pregnancy to teratogenic substances causes one or more structural abnormalities in the developing embryo or fetus. The discovery that a particular substance is a teratogen is usually the result of a significant increase in birth defects in an area or in a class of people.
The discovery of Minamata disease in Minamata Bay in postwar Japan is typical of the discovery of a teratogen. Minamata disease is a form of encephalopathy that closely resembles cerebral palsy. Eating fish contaminated with methyl mercury during pregnancy causes Minamata disease. The sudden appearance of over 3,000 cases of what appeared to be cerebral palsy puzzled health care officials. After an investigation it was discovered that Chisso Corporation, a chemical company, was dumping mercury into Minamata Bay where it had entered the food chain, including locally caught fish eaten by pregnant women. In the 1960s there was a surprising increase in the number of cases of phocomelia in Germany and Australia. An investigation identified thalidomide as a human teratogen. “Thalidomide babies” with severe birth defects were born to mothers who had been prescribed the drug thalidomide as a treatment for morning sickness. The exposure to the drug at the critical first trimester stage of development of the embryo produced severe birth defects.
The number of chemicals that are proven or suspected of being teratogens is growing as people are exposed to an increasing number of synthetic chemicals. In addition, attention to the problem has led to an increase in the number of defects identified. The identification of fetal warfarin syndrome, fetal hydantoin syndrome, fetal trimethadione syndrome, fetal alcohol syndrome, and low birth weight due to smoking during pregnancy, are fetal developmental problems that are given full recognition. Maternal infections during pregnancy from rubella or sexually transmitted diseases such as herpes simplex or syphilis can also cause teratogenicity. In addition, diabetes during pregnancy or other maternal health factors can interfere with fetal development.
Over-the-counter and prescription drugs are major sources of teratogenicity. Known teratogenic drugs are ACE inhibitors such as benazepril, captopril, and enalapril; antibiotics such as tetracycline and streptomycin; anti-depressants such as lithium; anticoagulants (blood-thinner) such as warfarin; or anticonvulsants. In addition, illegal drugs such as cocaine and marijuana are teratogenic substances. There are no absolute teratogens. The damaging effects of teratogenic substances are due to size of the dose, the length of exposure, and the stage of fetal development.
- M. Friedman and Janine E. Polifka, Teratogenic Effects of Drugs: A Resource for Clinicians (Teris) (Johns Hopkins University Press, 2000);
- James Schardein, Chemically Induced Birth Defects (Marcel Dekker, 2000);
- Thomas H. Shepard and Ronald Lemire, Catalog of Teratogenic Agents (Johns Hopkins University Press, 2004).