The FDA has provided these five categories to indicate the risk associated with the induction of birth defects. Unfortunately, they do not indicate the importance of when during gestation the medication is used, since some drugs are more dangerous during certain trimesters of pregnancy. The definitions provided below are, however, a useful tool in determining the possible risks associated with using the medication during pregnancy. Some newer medications may not yet have pregnancy classifications and are therefore not provided herein.

Controlled studies in women fail to demonstrate a risk to the fetus in the first trimester (and there is no evidence of a risk in later trimesters) and the possibility of fetal harm appears remote.

Either animal-reproduction studies have not demonstrated a fetal risk, but there are no controlled studies in pregnant women or animal-reproduction studies have shown an adverse effect (other than a decrease in fertility) that was not confirmed in controlled studies in women in the first trimester (and there is no evidence of a risk in later trimesters).

Either studies in animals have revealed adverse effects on the fetus (teratogenic or embryocidal, or other) and there are no controlled studies in women, or studies in women and animals are not available. Drugs should be given only if the potential benefit justifies the potential risk to the fetus.

There is positive evidence of human fetal risk, but the benefits from use in pregnant women may be acceptable despite the risk (e.g., if the drug is needed in a life-threatening situation or for a serious disease for which safer drugs cannot be used or are ineffective).