Does fetal sex influence the risk of venous thrombosis in pregnancy? A cohort study

Background: Venous thromboembolism is a prominent cause of maternal death.Objective: As inflammation is a well-known risk factor for venous thromboembolism and several studies have found a higher grade of inflammation in pregnancies bearing a male compared with female fetuses, we investigated the risk of pregnancy-related venous thromboembolism associated with sex of the fetus.Methods: This cohort study linked data from national registries and compared event rates and hazard ratios of venous thrombosis for pregnancies bearing a male fetus with those bearing a female fetus during pregnancy and in the first 3 months postpartum. National data from 1995 to 2017 were used. All Danish women aged 15 to 49 years with a live or stillbirth were eligible for inclusion; 1 370 583 pregnancies were included. Women with venous thrombosis, ischemic heart disease, cerebrovascular disease, thrombophilia, or cancer before conception were excluded.Results: The event rate for a venous thrombosis was 8.0 per 10.000 pregnancy years with a male fetus compared with 6.8 for a female fetus. The adjusted hazard ratio for venous thrombosis during pregnancies bearing a male was 1.2 (95% CI, 1.1-1.4), whereas in the postpartum period, it was 0.9 (95% CI, 0.7-1.0). The risk was elevated until week 30.Conclusion: These findings indicate a slightly greater risk of venous thrombosis during pregnancies bearing a male fetus than during pregnancies bearing a female fetus. There was no increased risk associated with fetal male sex in the postpartum period.