Venous Thromboembolism in Pregnant and Puerperal Women in Denmark 1995–2005: A National Cohort Study

Venous thromboembolism (VTE) occurs in about 1 per 1000 pregnancies. The risk of this serious and potentially fatal disorder is increased in pregnant women. There are conflicting data in the medical literature on how the risk of VTE changes during pregnancy and the puerperal period and on the magnitude of the risk according to gestational age. This historical controlled national cohort study compared the absolute and the relative risk of VTE at different gestational ages and in the postnatal period with the relative risk of VTE in nonpregnant women. The study was conducted over an 11-year period (1995–2005) in Denmark. Data were obtained through linkage of 4 National registries. Pregnant women with relevant diagnoses were identified in the National Registry of Patients, and women who used oral contraceptives were identified in the National Registry of Medical Products Statistics. The study population included Danish women who were 15 to 49 years of age during the study period. Of the 819,751 women pregnant at least once during the study period, 727 had a diagnosis of VTE. The absolute risk of VTE per 10,000 pregnant women years increased from 4.1 (95% confidence interval [CI]: 3.2–5.2) in gestational weeks 1–11 to 59.3 (95% CI: 46.1–76.4) in week 40. After delivery, the rate of VTE increased to a maximal rate of 60.0 (95% CI: 47.2–76.4) in the first week and then decreased to 2.1 (95% CI: 1.1–4.2) by 9 to 12 weeks. The incidence rate ratio increased from 1.5 (95% CI: 1.1–1.9) in gestational weeks 1 to 11 to 21.0 (95% CI: 16.7–27.4) in week 40 and 21.5 (95% CI: 16.8–27.6) in the first week of the puerperal period, subsequently falling to 3.8 (95% CI: 2.5–5.8) in 5 to 6 weeks after delivery. There was no apparent effect of maternal age or education on the risk of VTE. These findings show that the risk for VTE is strongly related to gestational age. The rate of VTE increases exponentially throughout pregnancy, reaching a maximum just after delivery. The data show no significant increase in risk by 6 weeks after delivery.