Identifying Teratogens

What studies are conducted to determine whether an agent is a teratogen?

In the past, there have been medications on the market that humans used that caused birth defects, such as thalidomide in the 1960s. Thalidomide was removed from the market after several babies were born with missing and malformed limbs to mothers who took thalidomide to relieve nausea and vomiting early in pregnancy. Thalidomide has recently been approved by the FDA for the treatment of certain diseases, but it is only available by a doctor's prescription. The following studies may be conducted to determine whether or not an agent is a teratogen:

  • Animal studies. Animal studies are the primary study method to determine whether a medication, or other environmental exposure, is safe during human pregnancy.

  • Observations from human exposure. Observations from human exposure (prior to federal regulations for drug testing) are another important way to determine whether a medication is a teratogen. The agent must be present during the critical stage of development.

How are medications rated for their safety?

The FDA has created five drug categories, to designate the safety of medications for their use during pregnancy, listed below. Always consult your doctor before taking any medication during pregnancy.




Medication has not shown an increased risk for birth defects in human studies.


Animal studies have not demonstrated a risk and there are no adequate studies in humans, OR animal studies have shown a risk, but the risk has not been seen in humans.


Animal studies have shown adverse effects, but no studies are available in humans, OR studies in humans and animals are not available.


Medications that are associated with birth defects in humans; however, there may be potential benefits in rare cases that outweigh their known risks.


Medications are contraindicated (should not be used) in human pregnancy because of known fetal abnormalities that have been demonstrated in both human and animal studies.

There have been thousands of medications studied over the years, and only about 30 to 40 have been found to have teratogenic effects in humans. It is always important, however, for a woman who is pregnant, or thinks she may be pregnant, to ask her doctor about the safety of a prescription medication or over-the-counter medication, vitamin, mineral, or herbal supplement before taking it.