Whenever possible, pregnant women should avoid drug therapy. The risks of taking drugs during pregnancy range from relatively minor fetal defects (such as ear tags or extra digits) to fetal death.

When drug therapy is considered, the drug’s benefits to the mother must be weighed against the risk to the fetus. Ideally, the drug should provide clear benefits to the mother without harming the fetus. To help prescribers and pregnant patients assess a drug’s risk-to-benefit ratio, the Food and Drug Administration assigns one of five pregnancy risk categories to each drug. In addition, certain drugs are not rated.

Category A: No evidence of risk exists. Adequate, well-controlled studies in pregnant women don’t show an increased risk of fetal abnormalities during any trimester.

Category B: The risk of fetal harm is possible but remote. Animal studies show no fetal risk; however, controlled studies haven’t been done in humans. Or animal studies do show a risk to the fetus, but adequate studies in pregnant women haven’t shown such a risk.

Category C: Fetal risk can’t be ruled out. Although animal studies show risks, adequate, well-controlled human studies are lacking. Despite the potential fetal risks, use of the drug may be acceptable because of benefits to the mother.

Category D: Positive evidence of fetal risk exists. Nevertheless, potential benefits from the drug may outweigh the risk. For example, the drug may be acceptable in a life-threatening situation or serious disease if safer drugs can’t be used or are ineffective.

Category X: Contraindicated during pregnancy. Studies in animals or humans or reports of adverse reactions show evidence of fetal risk that clearly outweighs any possible benefit to the patient.

Category NR: Not rated.