Understanding FDA pregnancy categories
The FDA introduced the A/B/C/D/X pregnancy category system in 1979 to help prescribers assess fetal risk. The system was replaced in 2015 with more detailed narrative labelling, but the letter categories remain the most commonly referenced framework in everyday clinical practice.
Category A: controlled studies show no risk
Adequate and well-controlled human studies have failed to demonstrate a fetal risk in any trimester. Very few drugs reach this standard, the most common examples are folic acid, levothyroxine at appropriate doses and some vitamins. Category A does not mean zero risk; it means the best available human evidence doesn't show any.
Category B: animal studies OK, limited human data
Either animal reproduction studies have not demonstrated a fetal risk but there are no controlled studies in pregnant women, or animal studies have shown an adverse effect that was not confirmed in controlled studies in women. Most widely used antibiotics, metformin, ondansetron and many other commonly used medications in pregnancy are Category B.
Category C: risk cannot be ruled out
Animal studies have shown adverse effects on the fetus and there are no adequate studies in women, or studies in women and animals are not available. Category C covers a huge range of drugs, from those with almost no risk to those with significant concerns. This is where clinical judgement matters most: many Category C drugs are used routinely and safely in pregnancy when the benefit outweighs the risk.
Category D: positive evidence of risk
There is positive evidence of human fetal risk based on adverse reaction reports or studies, but the potential benefits may warrant use of the drug despite potential risks. Category D drugs are used in pregnancy only when alternatives are inadequate or the indication is life-threatening. Examples include valproate, tetracyclines and ACE inhibitors in 2nd/3rd trimester.
Category X: contraindicated in pregnancy
Studies in animals or humans have demonstrated fetal abnormalities, or there is positive evidence of fetal risk based on adverse reaction reports, and the risks clearly outweigh any possible benefit. Category X drugs must not be used during pregnancy. Always confirm effective contraception before prescribing. Examples include isotretinoin, warfarin in 1st trimester, methotrexate and statins. Use our Pregnancy Drug Safety Checker for full guidance on any specific medication. For breastfeeding safety, see the Breastfeeding Drug Safety Checker.