Off-label drug use occurs when physicians prescribe a drug recommended for indications other than those listed on the prescription labeling. The purpose of the present investigation was to describe patterns of off-label drug use during pregnancy, including the types of drugs and their timing of use. All study pregnancies had to receive prenatal care at our state university clinics before 14 weeks' gestation and deliver at our institution during a five-month period. Prenatal records were reviewed, and subjects were interviewed after delivery. A total of 165 (22.6%) of the 731 eligible subjects took > or = 1 (average 1.7, 95% confidence interval 1.3-3.8) drugs for off-label indications. Nearly all drugs were taken for a short term during the third trimester. The primary purposes were to avoid an obstetric complication (premature labor and delivery, preeclampsia/eclampsia) or improve the capacity for eventual postnatal adaptation. Despite discussions with patients, on no occasion was it recorded on the chart that the patient was informed that the drug was recommended for an off-label indication. Results from this study should help drug manufacturers, insurance companies and federal regulatory agencies understand the common use of recommending certain drugs for off-label indications during pregnancy.