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

Rie Adser Virkus*, Anders P Mikkelsen, Øjvind Lidegaard, Christian Torp-Pedersen, Thomas Bergholt, Kenneth J Rothman, Ellen Løkkegaard

*Corresponding author for this work


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.

Original languageEnglish
JournalJournal of thrombosis and haemostasis : JTH
Issue number3
Pages (from-to)599-605
Number of pages7
Publication statusPublished - Mar 2023


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