Teratogens are drugs, chemicals, or even infections that can cause abnormal fetal development. There are billions of potential teratogens but only a few agents are proven to have teratogenic effects.

The majority of agents with which we come in contact are not proven teratogens. If you have a specific concern about a drug, chemical, or infection exposure during pregnancy, talk to your provider.

From a practical standpoint, the best way to reduce the risk of exposure to teratogens during pregnancy is to avoid taking medications when possible and to avoid the following types of exposures:

  • Excessive heat: Avoid prolonged stays in whirlpools, steam rooms, or saunas.
  • Herbal treatments: Ask your provider before you take any over-the-counter supplements during pregnancy. Products that claim to be "natural" are not necessarily safe during pregnancy.
  • Ionizing radiation: If a doctor orders a test with possible radiation exposure during your pregnancy, he or she must strongly believe that the risk of exposure is less than the risk of an untreated or undiagnosed condition. In most cases, the lower abdomen can be shielded to prevent exposure.
  • Children with runny noses, rashes, and fevers: It is not always possible to avoid sick children; fortunately, in the majority of these situations the exposures are inconsequential. Nonetheless, if at all possible, avoid such exposures while you are pregnant. Every parent knows that the surest place to acquire an illness is in a day care center or school. Certain infections like rubella, chickenpox, parvovirus (fifth disease), and cytomegalovirus (CMV) can be passed from child to adult. Adults are immune to many of these diseases. In some cases, however, the exposure may result in an infection that can affect the baby while in the uterus. If you have been exposed to a known viral illness, call your provider so that he can decide whether a blood test is warranted.
  • Toxoplasmosis: Toxoplasmosis is an infection transmitted from cat's feces to humans. If you are pregnant and have a cat you should minimize your exposure to kitty litter. If possible, have someone else clean the litter box. If you don't have anyone to help you with the litter, clean the litter box every day to minimize the risk of toxoplasmosis infection. You do not need to get rid of your cats.

In addition to avoiding these exposures, you should also avoid the following known teratogens:

  • angiotensin converting enzyme inhibitors-ACE inhibitors (for example, Zestril, Prinivil, and Lisinopril);
  • alcohol;
  • aminopterin;
  • androgens (Android, Methyltestosterone);
  • busulfan (Myleran);
  • carbamazepine (Tegretol);
  • chlorobiphenyls;
  • coumarins (Coumadin, warfarin);
  • cyclophosphamide (Cytoxan);
  • danazol (Danocrine);
  • diethylstilbestrol (DES);
  • etretinate (Tegison);
  • isotretinoin (Accutane);
  • lithium (Eskalith);
  • methimazole (Tapazole);
  • methotrexate (Rhumatrex);
  • penicillamine (Depen, Cuprimine);
  • phenytoin (Dilantin);
  • radioactive iodine;
  • tetracycline (Sumycin);
  • trimethadione (Tridione); and
  • valproic acid (Depakene).

Some of these agents are easy to avoid. Others may be required for a medical condition and are unavoidable. For example, a woman with epilepsy may require phenytoin to control her seizures. Despite the risk of teratogenic effects, the woman is better off taking the phenytoin than risking uncontrolled seizures during pregnancy.

If you require treatment with any of the agents listed above, ask your health care provider to send you to a geneticist. Geneticists have expertise in the effects of teratogens on the fetus and can help you determine your actual risk given a specific exposure. You may also receive a targeted ultrasound evaluation looking for evidence that the fetus has in some way been affected.