Postpartum Venous Thrombosis in Sweden: A Nationwide Family Study

Postpartum venous thromboembolism (VTE) is a serious complication. The importance of family history of VTE remains unclear in the postpartum period.

To assess the importance of family history of VTE for postpartum VTE among primiparous women without previous VTE.

This nationwide population-based cohort study included 856,380 Swedish primiparous women between 1992-2018, without a history of VTE, followed for incident postpartum VTE within 90 days after delivery. Relatives were identified through the Swedish Multi-Generation Register. The exposure was family history of VTE in grandparents, parents, full siblings, half siblings, cousins, and spouses. A hereditary score consisting of four categories was constructed based on affected biological relatives and their degree of genetic relatedness. VTE in a grandparent contributed 0.5 points, VTE in a parent 1 point, VTE in a full sibling 1 point, VTE in a half sibling 0.5 points, and VTE in a cousin 0.25 points. Reference category 0=0 points, category 1=0.25-1 points, category 2=1.25-1.5, and category 3 ≥1.75 points. Cox proportional hazards models with multivariable adjustment were used to estimate hazard ratios (HRs) with 95% confidence intervals (CI).

A total of 788 women (0.09%) developed postpartum VTE. Family history in grandparents (HR=1.41, 95% CI 1.21-1.64), parents (HR=2.03, 95% CI 1.60-2.56), full sibling (HR=2.55, 95% CI 1.53-4.26), and half siblings (HR=2.28, 95% CI 1.08-4.80) was associated with increased VTE risk. The hereditary score showed a clear dose-response pattern compared with those without family history: the fully adjusted HR was 1.50 (95% CI 1.29-1.74) for the hereditary score category 1, HR=2.89 (95% CI 2.01-4.16) for the score category 2, and HR=3.66 (95% CI 2.06-6.51) for the score category 3. Additional independent predictors for VTE in the fully adjusted model were obesity, cancer, systemic lupus erythematosus, varicose veins, gestational hypertension, preeclampsia or eclampsia, placental abruption, endometritis or puerperal fever, caesarean delivery, and postpartum hemorrhage-blood transfusion status.

Family history of VTE is a dose-graded risk factor for postpartum VTE, with the risk increasing according to the number of affected relatives and degree of genetic relatedness.