The expert answers: The US Food and Drug Administration (FDA) has a five-level categorization system using the letters A, B, C, D, and X to describe a drug's potential to harm an unborn child. Drugs with a pregnancy category rating of "A" or "B" are preferred for use during pregnancy because there have been studies that suggest no risk, while those with an "X" rating should absolutely be avoided. Category "C" drugs have not been studied sufficiently for experts to conclude whether they are safe during pregnancy, and category "D" refers to drugs that can be harmful but may have medical benefits that outweigh the risks.
Controlled studies in women fail to demonstrate a risk to the fetus in the first trimester (and there is no evidence of a risk in later trimesters), and the possibility of fetal harm appears remote.
Either (1) animal-reproduction studies have not demonstrated a fetal risk, but there are no controlled studies in pregnant women; or (2) animal-reproduction studies have shown an adverse effect (other than a decrease in fertility) that was not confirmed in controlled studies in women in their first trimester (and there is no evidence of a risk in later trimesters).
Either (1) studies in animals have revealed adverse effects on the fetus (teratogenic or embryocidal or other) and there are no controlled studies in women, or (2) studies in women and animals are not available. These drugs should be given only if the potential benefits justify the potential risk to the fetus.
There is evidence of risk to the unborn child, but the benefits from use in pregnant women may be acceptable despite the risk (for example, if the drug is needed in a life-threatening situation, or if safer drugs cannot be used or are ineffective for a serious disease).
Studies in animals or human beings have demonstrated fetal abnormalities, or there is evidence of fetal risk based on human experience, and the risk of the use of the drug in pregnant women clearly outweighs any possible benefit. Women who are or may become pregnant should not use these drugs.
These risk factors are designed to help clinicians and patients quickly classify a drug for use during pregnancy. However, any decision to continue or discontinue medications during pregnancy must weigh the potential benefits to the mother against the possible risks to the unborn child. Before using any prescription, nonprescription, or natural remedy during pregnancy, please consult your doctor or obstetrician to discuss the safety of the medication for you and your unborn child.