The objective of this study was to assess the utilization of postpartum thromboprophylaxis with heparin in patients according to the Royal College of Obstetrics and Gynaecology Green-Top guidelines after change from an opt-in to an opt-out policy for health care providers ordering heparin thromboprophylaxis after cesarean delivery. The present study is a retrospective review of 500 consecutive births at one academic institution before and after implementation of a uniform thromboprophylaxis policy with heparin for all cesarean deliveries. An "opt-out" policy for ordering physicians was implemented by automatically defaulting to order heparin in the electronic order set used after cesarean delivery. Cesarean delivery rates were similar during both time periods. Heparin thromboprophylaxis was indicated in 99.6% of the cesarean delivery population before implementation and 94.5% after implementation. Prior to implementation only 5.7% received thromboprophylaxis compared to 96.1% after implementation, P<0.0001. An opt-out heparin thromboprophylaxis policy improves compliance with thromboprophylaxis guidelines compared to an opt-in policy. Institutions should consider opt-out heparin thromboprophylaxis policies after cesarean deliveries to improve compliance with recommendations.