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. These effects can result in a baby being born with a birth defect. Approximately 4 to 5 percent of birth defects are caused by exposure to a teratogen.

The majority of agents that people come into contact with aren’t proven teratogens. Talk to your doctor if you’re concerned about being exposed to a particular medication, chemical, or infection during pregnancy.

Researchers believe a teratogen can affect a developing fetus about 10 to14 days after conception.

The best way to reduce the risk of exposure to teratogens during pregnancy is to avoid taking medications when possible and to avoid exposure to the following:

Excessive Heat

Avoid prolonged stays in whirlpools, steam rooms, or saunas.

Herbal Treatments

Talk to your doctor before you start taking any over-the-counter supplements during pregnancy. Products that claim to be natural may not necessarily be safe to use during pregnancy.

Ionizing Radiation

If your doctor orders a test with possible radiation exposure during your pregnancy, they 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 with a protective apron to prevent exposure.

Children with Runny Noses, Rashes, and Fevers

It isn’t always possible to avoid sick children, and in most cases, exposures only lead to minor illnesses. When you can, it’s best to avoid such exposures while you’re pregnant. Every parent knows that the easiest place to acquire an illness is in a day care center or school, so avoid these locales as much as possible.

Certain infections can be passed from child to adult, including chickenpox, rubella, and cytomegalovirus (CMV). 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 womb. If you’ve been exposed to a known viral or bacterial illness, call your doctor right away so they can decide whether a blood test is needed.


Toxoplasmosis is an infection that can be transmitted from cat feces to humans. If you’re pregnant and have a cat, you should minimize your exposure to the litter box as much as possible. Ask someone else in your household to clean the litter box. If you don’t have anyone to help you with the litter, clean the litter box every day to reduce your risk of contracting toxoplasmosis. You don’t need to get rid of your cat.

Known Teratogens

You should also avoid known teratogens. These include

  • angiotensin converting enzyme (ACE) inhibitors, such as Zestril and Prinivil
  • alcohol
  • aminopterin
  • androgens, such as methyltestosterone (Android)
  • busulfan (Myleran)
  • carbamazepine (Tegretol)
  • chlorobiphenyls
  • cocaine
  • coumarins
  • warfarin (Coumadin)
  • cyclophosphamide (Cytoxan)
  • danazol (Danocrine)
  • diethylstilbestrol (DES)
  • etretinate (Tegison)
  • isotretinoin (Accutane)
  • lead
  • lithium (Eskalith)
  • mercury
  • methimazole (Tapazole)
  • methotrexate (Rheumatrex)
  • penicillamine (Depen, Cuprimine)
  • phenytoin (Dilantin)
  • phenobarbital (Solfoton)
  • propylthiouracil (PTU)
  • prostaglandins
  • radioactive iodine
  • tetracycline (Sumycin)
  • tobacco
  • trimethadione (Tridione)
  • valproic acid (Depakene)

Some of these agents are easy to avoid. Others may be required for a medical condition and are unavoidable. For instance, you may require phenytoin to control your seizures if you’re pregnant and you have epilepsy. Despite the risk of teratogenic effects, you may be better off taking the phenytoin than risking the occurrence of uncontrolled seizures during pregnancy.

If you need treatment with any of the medications that are known teratogens and you’re pregnant, ask your doctor to send you to a geneticist. Geneticists specialize in the effects of teratogens on fetuses and can help you assess your actual risk given a specific exposure. You may also receive a targeted ultrasound evaluation to determine whether the fetus has been affected in some way.