Drug Categories for Pregnancy
Determined by the FDA
AControlled human studies show no risk. Controlled studies in pregnant wormsen fail to demonstrate a risk to the fetus in the first trimester with no evidence of risk in later trimesters. The possibility of fetal harm seems remote.
BNo evidence of risk in studies. Either animal reproduction studies have not demonstrated a fetal risk but these are no controlled studies in pregnant women, or animal reproduction studies have shown an adverse effect (other than decreased 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.
CRisk cannot be ruled out. Either studies in animals have revealed adverse effects on the fetus (teratogenic or embryocidal effects 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 benefits justify the potential risk to the fetus.
DPositive evidence of risk. There is positive evidence of human fetal risk, but the benefits from use in pregnant women may be acceptable despite the risk (example is 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.
XContraindicated in pregnancy. Studies in animals or humans have demonstrated abnormalities or there is evidence of fetal risk based on human experience, or both, and the risk of the use of the drug in pregnant women clearly outweighs any possible benefit. The drug is contraindicated in women who are or may become pregnant
Note that in 2015 the FDA began to change the rules and phase out the use of A B C D X ratings on drugs. Instead they require information from available human and animal studies of 1) known or potential maternal or fetal adverse reactions and 2) dose adjustments needed during pregnancy and postpartum period. Additional information at the FDA Pregnancy and Lactation Final rule