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Background: The risk for venous thromboembolism during pregnancy or postpartum is uncertain. Objectives: To estimate the relative and absolute risk for deep venous thrombosis and pulmonary embolism during pregnancy and postpartum and to describe trends in incidence. Design: Population-based inception cohort study using the resources of the Rochester Epidemiology Project. Setting: Olmsted County, Minnesota. Patients: Women with deep venous thrombosis or pulmonary embolism first diagnosed between 1966 and 1995, including women with venous thromboembolism during pregnancy or the postpartum period (defined as delivery of a newborn no more than 3 months before the deep venous thrombosis or pulmonary embolism event date, including delivery of a stillborn infant after the first trimester). Measurements: The authors obtained yearly counts of live births in Olmsted County between 1966 and 1995 from the Minnesota Department of Health. Results: The relative risk (standardized incidence ratio) for venous thromboembolism among pregnant or postpartum women was 4.29 (95% CI, 3.49 to 5.22; P< 0.001), and the overall incidence of venous thromboembolism (absolute risk) was 199.7 per 100000 woman-years. The annual incidence was 5 times higher among postpartum women than pregnant women (511.2 vs. 95.8 per 100 000), and the incidence of deep venous thrombosis was 3 times higher than that of pulmonary embolism (151.8 vs. 47.9 per 100 000). Pulmonary embolism was relatively uncommon during pregnancy versus the postpartum period (10.6 vs. 159.7 per 100 000). Over the 30-year study period, the incidence of venous thromboembolism during pregnancy remained relatively constant whereas the postpartum incidence of pulmonary embolism decreased more than 2-fold. Limitations: Because the Olmsted County population was 98% white and of non-Hispanic ethnicity, the results may not be generalizable to other ethnicities. Conclusions: Among pregnant women, the highest risk period for venous thromboembolism and pulmonary embolism in particular is during the postpartum period. Any prophylaxis against these events should be particularly targeted to postpartum women. Although the incidence of pulmonary embolism has decreased over time, the incidence of deep venous thrombosis remains unchanged, indicating the need to better identify pregnant women at increased risk.