Identifying teratogens

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

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. In the past, there have been medications on the market that humans used which caused birth defects, such as thalidomide in the 1960's. Thalidomide was removed from the market after several babies were born with missing and malformed limbs to mothers who took thalidomide early in pregnancy. Thalidomide has recently been approved by the FDA for the treatment of certain diseases, but it is only available by a physician's prescription.
  • 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 official criteria for determining whether an agent is teratogenic in humans was determined by Koch in 1971. They are known as "Koch's Postulates":
    1. The agent must be present during the critical stage of development.
    2. The agent produces a particular pattern of birth defects in animal studies.
    3. The agent crosses the placenta and there is a dose-response relationship.
    4. There is an abrupt increase in the frequency of a particular defect or group of defects (syndrome).
    5. The increase of defects is associated with the use of a new drug or the widespread exposure to a chemical or environmental change.
    6. There is an absence of other factors to explain the observations.
    7. The mechanism of teratogenesis makes biological sense.

How are medications rated for their safety?

Prior to 1980, most medications included literature that stated that their use in pregnancy had not been approved. The US Food and Drug Administration (FDA) has since created five drug categories, to designate the safety of medications for their use during pregnancy, including the following. Always consult your physician before taking any medication during pregnancy.

Category  Description 


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 thinks she could be pregnant, or is pregnant, to ask her physician about the safety of a medication or over-the-counter medication, vitamin, mineral, or herbal supplement before taking it.

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