The FDA divides drugs into five pregnancy risk categories, or classes.  The five classes are A,B,C,D, and X.  The meaning of the classification system is as follows:

• Class A - Adequate, well-controlled studies in pregnant women have not shown an increased risk of fetal abnormalities to the fetus in any trimester of pregnancy.
• Class B - Animal studies have revealed no evidence of harm to the fetus, however, there are no adequate and well-controlled studies in pregnant women.
  OR
  Animal studies have shown an adverse effect, but adequate and well-controlled studies in pregnant women have failed to demonstrate a risk to the fetus in any trimester.
• Class C - Animal studies have shown an adverse effect and there are no adequate and well-controlled studies in pregnant women.
  OR
  No animal studies have been conducted and there are no adequate and well-controlled studies in pregnant women.
• Class D - Adequate well-controlled or observational studies in pregnant women have demonstrated a risk to the fetus.
  However the benefits of therapy may outweigh the potential risk.  For example, the drug may be acceptable if needed in a life-threatening situation or serious disease for which safer drugs cannot be used or are ineffective.
• Class X - Adequate well-controlled or observational studies in animals or pregnant women have demonstrated positive evidence of risk of fetal abnormalities or risks.