Women of childbearing age in the United States should be immune to measles, rubella, mumps, tetanus, diphtheria, and, through childhood immunization, poliomyelitis. Of particular importance are rubella, because of the devastating effects of congenital infection; measles, because of its high risk for causing spontaneous abortion, preterm birth, and maternal morbidity; and tetanus, because transplacental transfer of antitoxin prevents neonatal tetanus. Protection against hepatitis B is critical for the pregnant woman and newborn who are at high risk for infection. Combined tetanus and diphtheria toxoids are the only immunobiologic agents routinely indicated for susceptible pregnant women. Other vaccines may be indicated for pregnant women under special circumstances. Measles, mumps, and rubella single-antigen vaccines, as well as the combined vaccine, are contraindicated in pregnancy but should ideally be given preconceptionally or postpartum. In the decision of whether to immunize a pregnant woman with other vaccines, the risk for exposure to disease and its deleterious effects on the pregnant woman and her fetus must be balanced against the efficacy of the vaccine and any possible deleterious effects resulting from it. Preconception immunization of women to prevent disease in the offspring, when practical, is preferred to vaccination of pregnant women.