The purpose of this study was to provide information on the prevalence of the use of prescription drugs among pregnant women in the United States. A retrospective study was conducted with the use of the automated databases of 8 health maintenance organizations that are involved in the Health Maintenance Research Network Center for Education and Research on Therapeutics. Women who delivered of an infant in a hospital from January 1, 1996, through December 31, 2000, were identified. Prescription drug use according to therapeutic class and the United States Food and Drug Administration risk classification system was evaluated, with the assumption of a gestational duration of 270 days, with three 90-day trimesters of pregnancy, and with a 90-day period before pregnancy. Nonprescription drug use was not assessed. During the period 1996 through 2000, 152,531 deliveries were identified that met the criteria for study. For 98,182 deliveries (64%), a drug other than a vitamin or mineral supplement was prescribed in the 270 days before delivery: 3595 women (2.4%) received a drug from category A; 76,292 women (50.0%) received a drug from category B; 57,604 women (37.8%) received a drug from category C; 7333 women (4.8%) received a drug from category D, and 6976 women (4.6%) received a drug from category X of the United States Food and Drug Administration risk classification system. Overall, 5157 women (3.4%) received a category D drug, and 1653 women (1.1%) received a category X drug after the initial prenatal care visit. Our finding that almost one half of all pregnant women received prescription drugs from categories C, D, or X of the United States Food and Drug Administration risk classification system highlights the importance of the need to understand the effects of these medications on the developing fetus and on the pregnant woman.