A teratogen is any agent that can cause birth defects if a fetus is exposed to it. Teratogens are usually drugs or infectious agents such as bacteria or viruses, and can affect a fetus from as early as the first few weeks after conception through the second trimester. For this reason, ideally, women should avoid all medications during pregnancy. However, in some circumstances there are medical risks and benefits that must be weighed, particularly if a medication is important to the health of the prospective mother.
To properly assess the potential danger of a teratogen, information is required about its effect on embryonic development, its ease of passage across the placenta, and the dosage and timing of fetal exposure to the teratogen. There are, however, limitations on the ability of doctors to predict the risk of birth defects arising from fetal exposure to any particular drug because of the lack of information on the effects of multiple medication use and possible drug interactions, the inability to control for other exposures women may have during pregnancy, and the unique genetic susceptibilities of each person. In addition, there are limited clinical studies that address this problem.
Risks for birth defects or adverse pregnancy outcomes associated with any type of exposure are in addition to the 3 percent background risk for birth defects in all pregnancies. There is no evidence linking paternal exposures to teratogenicity for the developing fetus, though exposure to some agents can reduce male fertility.
One well-documented teratogen is the drug thalidomide, which was taken by tens of thousands of women in the 1950s and early 1960s to treat nausea during pregnancy, before its potent teratogenic effects were recognized. Even a single dose caused severe birth defects, including amelia (absence of limbs), phocomelia (short limbs), incomplete or absent bone growth, ear and eye abnormalities, congenital heart defects, and others.
Another potent teratogen is a substance called isotretinoin, marketed under the brand name of Accutane, used to treat severe acne. Birth defects following prenatal exposure include serious central nervous system defects such as hydrocephalus, microcephaly, and mental retardation, as well as cleft lip and palate and anomalies of cardiovascular, limb, eye, and other systems. For women who have taken this substance, it is recommended to delay pregnancy at least one month after they have stopped using it.
Like medications, "recreational" drugs such as alcohol and cocaine can act as teratogenic agents. Fetal alcohol syndrome (FAS) and fetal alcohol effects (FAE) are the most common, completely preventable, and potentially devastating disorders caused by alcohol use during pregnancy. FAS is one of the leading causes of mental disabilities in children. Fetal exposure to alcohol creates additional problems in children born with FAS/FAE, such as characteristic facial features, growth retardation, central nervous system difficulties, learning disabilities, and behavioral problems. No amount of alcohol is thought to be safe during pregnancy; however, some of its effects may be prevented by stopping the exposure during or shortly after the first trimester.
Cocaine use during pregnancy is known to increase the risk of miscarriages and premature labor and delivery. Disturbances in the behavior of exposed newborns have been reported, such as irritability, irregular sleeping patterns, muscular rigidity, and poor feeding. Some birth defects associated with the use of this drug include urinary and genital malformations, as well as defects of the limbs, intestines, and the skull.
Conditions arising from infectious teratogenic agents include toxoplasmosis, syphilis, and rubella. In each of these cases, the mother is exposed to the infectious agent, then transmits it to the fetus. In toxoplasmosis, the parasite Toxoplasma gondii can be transmitted from cats to humans through contact with cat feces (cleaning litter box or gardening), or through consumption of undercooked meats, poorly washed fruits and vegetables, goat's milk, or raw eggs. Mother-to-fetus transmission is more likely if maternal infection occurs in the last few weeks before delivery, but early fetal exposure is generally associated with greater severity of defects in the child. Overall, 20 to 30 percent of untreated, infected newborns have birth defects, including seizures, microcephaly, and other severe effects on the nervous system. Treatment of the mother with antibiotics during pregnancy is safe for the fetus, and significantly reduces the likelihood of fetal infection.
Syphilis is an infection caused by the spirochete Treponema pallidum. This bacterium crosses the placenta and may result in fetal infection. If untreated, the pregnancy may end in miscarriage, stillbirth, or neonatal death. Signs of congenital infection include jaundice, joint swelling, rash, anemia, and characteristic defects of bone and teeth. Maternal treatment of this condition may help prevent the transmission to the fetus and its ill effects.
Rubella is the scientific name for the disease commonly known as German measles. Congenital rubella syndrome (CRS) results from the exposure of an unprotected pregnant woman to the rubella virus, and can lead to major birth defects, including serious malformations of the heart, blindness, deafness, and mental retardation. CRS has been virtually eradicated in the United States because of the near-universal vaccination against rubella, now part of the standard childhood immunizations program. Unfortunately, this vastly improved situation is not as common in much of the rest of the world.