BACKGROUND: Opioid use has increased dramatically in North America. The safety of opioids in pregnancy is uncertain, but they are associated with several fetal abnormalities and contribute to rising rates of neonatal abstinence syndrome. We examined opioid use before and during pregnancy in a complete population-based cohort. METHODS: We examined opioid use in a cohort of all pregnant women in Manitoba, Canada, from 2001 to 2013. Opioid use was defined by prescriptions for opioids, converted to oral morphine equivalents (MEQ), during the 3 months before pregnancy and for each trimester. Given that the exposure per person may vary (because not all women complete all time periods), we determined a weighted number of pregnancies in each period. RESULTS: During the study period, 174 848 completed pregnancies were eligible for analysis (173 680 live births and 1168 stillbirths and intrauterine deaths), which represented a weighted value of 175 174 pregnancies. Among these pregnancies, 6.7% of the women filled opioid prescriptions in the 3 months before pregnancy. Use declined to 4.2% during the first trimester and further declined to 3.0% and 2.9% in the second and third trimesters, respectively. Over the study period, there was a modest increase in opioid use overall (from 7.3% to 7.7%). MEQ did not decline during pregnancy, and the mean MEQ increased significantly over the study period (from 284 mg to 1218 mg). Prescriptions for codeine were filled by 96.9% of the users, accounting for 66.2% of MEQ. INTERPRETATION: Although many of the women using opioids before pregnancy discontinued or reduced use of these drugs during pregnancy, the volume of opioids consumed by those who continued opioid use did not decline during pregnancy. The increasing dosage and increased use of higher-potency opioids by pregnant women highlights the need for continued evaluation of and education about the benefits and risks of this practice.