BACKGROUND: Owing to the paucity of evidence available on the risks and benefits of drug use in pregnancy, the use of prescription medicines is a concern for both pregnant women and their health care providers. OBJECTIVE: The aim of this study was to measure the frequency, timing, and type of medicines used before, during, and after pregnancy in a Canadian population. METHODS: This retrospective cohort analysis used population-based health care data from all pregnancies ending in live births in hospitals in British Columbia from April 2001 to June 2006 (n = 163,082). Data from hospital records were linked to those in outpatient prescription-drug claims. Data from prescriptions filled from 6 months before pregnancy to 6 months postpartum were analyzed. Drugs were classified by therapeutic category and US Food and Drug Administration (FDA) pregnancy risk categories. RESULTS: Prescriptions were filled in 63.5% of pregnancies. Evidence on safety is limited for many of the medicines most frequently filled in pregnancy, including codeine, salbutamol, and betamethasone. At least 1 prescription for a category D or X medicine was filled in 7.8% of pregnancies (5.5% category D; 2.5% category X). The most frequently filled prescriptions for category D drugs were benzodiazepines and antidepressants. The most frequently filled prescriptions for category X drugs were oral contraceptives and ovulation stimulants filled in the first trimester. CONCLUSIONS: The majority of pregnant women in British Columbia filled at least 1 prescription, and ~1 in 13 filled a prescription for a drug categorized as D or X by the FDA. The prevalence of maternal prescription drug use emphasizes the need for postmarketing evaluation of the risk-benefit profiles of pharmaceuticals in pregnancy. Future research on prenatal drug use based on administrative databases should examine maternal treatment adherence and the determinants of maternal drug use, considering maternal health status, sociodemographics, and the characteristics and providers of prenatal care.