Drugs in pregnancy are categorized by the U.S. Food and Drug Administration (FDA) into category A, B, C, D, or X, depending on how much evidence there is of potential harm to the fetus.
Category A: Adequate studies in pregnant women have not demonstrated a risk to the fetus in the first trimester, and there is no evidence of risk in later trimesters. The possibility of fetal harm appears remote. Example: Vitamin C, when used in recommended daily allowance dosage
Category B: Animal studies have not demonstrated fetal risk but there are no adequate studies in pregnant women, or animal studies have shown an adverse effect (other than a decrease in fertility), but adequate studies in pregnant women have not demonstrated a risk to the fetus during the first trimester and there is no evidence of risk in later trimesters). Example: Ampicillin.
Category D: Positive evidence of human fetal risk exists, but the potential benefits from the use of the drug in pregnant women may be acceptable despite these risks (for a life-threatening condition or a serious disease for which safer drugs cannot be used or are ineffective). Example: Phenytoin (anticonvulsive).
Category X: Studies in animals or humans demonstrate fetal abnormalities or adverse-reaction reports indicate evidence of fetal risk. The risk of use in a pregnant woman clearly outweighs any possible benefit. Example: Isotretinoin