Risk factors for venous thromboembolism in pregnant women in Japan: a retrospective study

Venous thromboembolism (VTE) is a leading cause of maternal mortality in the developed countries, including Japan, where nationwide data are lacking. This study aimed to clarify the incidence and key risk factors for VTE among pregnant and postpartum women in Japan using a large perinatal registry database. We retrospectively enrolled 146,791 pregnant women registered in the Japan Society of Obstetrics and Gynecology perinatal registry database between January and December 2020 and divided them into two groups based on the presence or absence of VTE. The incidence of VTE was determined, and odds ratios for VTE were calculated based on various maternal parameters and obstetric factors. The incidence of VTE was 1/1000. Univariate analysis revealed that age ≥35 years, cesarean section, blood loss, preterm delivery before 37 weeks, a history of VTE, fetal growth restriction, and multiple gestations were risk factors for VTE. Multivariate analysis revealed that independent risk factors for VTE included a cesarean section (odds ratio [OR]: 2.1, 95% confidence interval [CI]: 1.4-3.0, p < 0.001), preterm delivery before 37 weeks (OR: 1.6, 95% CI: 1.1-2.5, p = 0.01), and history of VTE (OR: 62.1, 95% CI: 31.2-120, p < 0.001). A History of VTE, cesarean section, and preterm delivery before 37 weeks are significant risk factors for VTE during pregnancy. The identified risk factors could prove valuable in preventing VTE after pregnancy and delivery, particularly in Japan.