Incidence and prognosis of superficial vein thrombosis during pregnancy and the post-partum period: a Danish nationwide cohort study

Background: The incidence of superficial vein thrombosis (SVT) of the legs and the subsequent risk of venous thromboembolism during pregnancy and the post-partum period is unknown. To better understand the clinical course of SVT during these times, we aimed to estimate the incidence rate of SVT during pregnancy and in the post-partum period, as well as the risk of subsequent venous thromboembolism. Methods: In this nationwide cohort study, we collected data on all pregnant women who delivered between Jan 1, 1997, and Dec 31, 2017, in Denmark were extracted from the Danish Medical Birth Register, the Danish National Patient Registry, and the Danish National Prescription Registry. Data on ethnicity were not available. Incidence rates per 1000 person-years were calculated for each trimester and the antepartum and post-partum period. Among women with a pregnancy-related SVT, risk of subsequent venous thromboembolism within the same pregnancy or post-partum period were calculated and compared with a matched cohort of pregnant women without SVT using Cox proportional hazards analysis. Findings: During 1 276 046 deliveries, 710 diagnoses of lower extremity SVT occurred from conception up to 12 weeks postpartum (0·6 per 1000 person-years [95% CI 0·5–0·6]). The incidence rates of SVT per 1000 person-years were 0·1 (95% CI 0·1–0·2) during the during the first trimester, 0·2 (0·2–0·3) during the second trimester, and 0·5 (0·5–0·6) during the third trimester. The incidence rate was 1·6 per 1000 person-years (95% CI 1·4–1·7) during the post-partum period. Of the 211 women with antepartum SVT included in the analysis, 22 (10·4%) were diagnosed with venous thromboembolism, compared with 25 (0·1%) in women without SVT (hazard ratio 83·3 [95% CI 46·3–149·7]). Interpretation: The incidence rate of SVT during pregnancy and the post-partum period was low. However, if SVT during pregnancy was diagnosed, the risk of developing venous thromboembolism during the same pregnancy was high. These results might help physicians and patients to make decisions about anticoagulant management of pregnancy-related SVT. Funding: None.