20. U.S. Food and Drug Administration Classification of Teratogenic Drug Risk Adapted from Federal Register(1979) Contraindicated in Pregnancy. X Positive human evidence of fetal risk. D Risk cannot be ruled out. C No evidence of risk in humans. Either animal studies show no fetal risk or animal studies show risk but human studies do not show risk B Controlled studies show no risk. Well controlled human studies failed to demonstrate risk to fetus A Description of Risk Category