Trends in the Incidence of Venous Thromboembolism During Pregnancy or Postpartum: A 30-Year Population-Based Study

Abstract

It is necessary to have an idea of the incidence and relative risk of venous thromboembolism in pregnant women and in the postpartum period to identify those who may benefit from prophylaxis. Previously reported rates differ widely. This population-based study was undertaken to determine the relative risk and incidence of deep venous thrombosis (DVT) and pulmonary embolism (PE) within a defined area of Minnesota in the years 1966-1995. Incidence rates were estimated during pregnancy and in the first 3 months postpartum. The relative risk, or standardized incidence ratio, for venous thromboembolism in pregnant or postpartum women was 4.29 (95% confidence interval, 3.49-5.22; P <.001), and its overall incidence, or absolute risk, was 199.7 per 100,000 woman-years of follow up. The annual incidence was five times higher in postpartum women than in pregnant women (511.2 vs. 95.8 per 100,000). The incidence of DVT was three times higher than that of PE (151.8 vs. 47.9 per 100,000). PE was relatively infrequent during pregnancy compared with the postpartum period (10.6 vs. 159.7 per 100,000). The incidence of venous thromboembolism during pregnancy remained relatively constant during the 30-year study period. The incidence of PE in the postpartum period, however, decreased by more than twofold. Venous thromboembolism during pregnancy was most frequent in the youngest women (15 to 19 years of age); DVT without PE predominated. In the postpartum period, the incidence of venous thromboembolism increased with advancing maternal age and was most frequent in those aged 35 and older. PE was substantially more frequent in the oldest postpartum women. Women who become pregnant appear to be most vulnerable to venous thromboembolism, and to PE in particular, during the postpartum period, suggesting that prophylactic measures be taken at this time. DVT, unlike PE, has not decreased over the years, emphasizing the need to more reliably identify those pregnant women who are at increased risk.