Women who are pregnant or considering pregnancy are rightly concerned about whether the medicines they take might cause birth defects or other harm to their developing baby. Doctors are looking for the same kind of information when choosing the best medications to prescribe for young women. In the United States, the Food and Drug Administration (FDA) is one source of guidance. The FDA looks at the evidence available about the risk of harm if each medication is used during pregnancy. Then it assigns each one to a "Pregnancy Category" (A, B, C, D, or X). The category for each medication is reported in its package insert.

Here's a list of the five FDA categories:

	Category		Description
	A		Adequate, well-controlled studies in pregnant women have not shown any risk to the fetus in the first three months of pregnancy, and there's no evidence of later risk either. Very few medications have been tested to this level.
	B		There have been no adequate, well-controlled studies in women but studies using animals have not found any risk to the fetus, or animal studies have found risk that was not confirmed by adequate studies in pregnant women. Not many adequate studies have been performed in pregnant women, so the first situation (not enough information) usually applies if a medication is assigned to this category.
	C		There have been no adequate, well-controlled studies in women, but studies using animals have shown a harmful effect on the fetus, or there haven't been any studies in either women or animals. Caution is advised, but the benefits of the medication may outweigh the potential risks.
	D		There is clear evidence of risk to the human fetus, but the benefits may outweigh the risk for pregnant women who have a serious condition that cannot be treated effectively with a safer drug.
	X		There is clear evidence that the medication causes abnormalities in the fetus. The risks outweigh any potential benefits for women who are (or may become) pregnant.

This system, while the best now available, could be interpreted as suggesting clear differences between medications in various categories when the differences are really much less distinct. The categorization of many medications is based mainly on lack of evidence one way or the other. The FDA is considering a change to more detailed descriptions that would also include information on fertility and breastfeeding.

Topic Editor:Steven C. Schachter, M.D.
Last Reviewed:10/23/03