US FDA Pregnancy Category Definitions

A : Adequate, well-controlled studies in pregnant women fail to demonstrate a risk to the fetus in the first (second, third, or all) trimesters, and the possibility of fetal harm appears remote.

B : Animal studies do not indicate a risk to the fetus; however, there are no adequate, well-controlled studies in pregnant women. OR animal studies have shown an adverse effect on the fetus but adequate, well-controlled studies in pregnant women have failed to demonstrate a risk to the fetus. Despite the animal findings, the possibility of fetal harm appears remote, if used during pregnancy.

C : Animal studies have shown that the drug exerts teratogenic or embryocidal effects, and there are no adequate, well-controlled studies in pregnant women, OR no studies available in either animals or pregnant women.

D : Positive evidence of human fetal risk exists, but benefits in certain situations (eg, life-threatening situations or serious disease for which safer drugs cannot be used or are ineffectively) may make use of the drug acceptable despite its risks.

X : Studies in animals or humans have demonstrated fetal abnormalities or there is positive evidence of fetal risk based on human experience, or both, and the risk clearly outweight any possible benefit. The drug is contraindicated in women who are or may become pregnant.