Design: Nationwide population-based cohort, 1992 to 2018
Population: 856,380 Swedish primiparous women without previous VTE
Primary outcome: VTE within 90 days of delivery
Effect: Parent HR 2.03 (95% CI 1.60 to 2.56); highest hereditary score HR 3.66 (2.06 to 6.51)

A Swedish nationwide cohort followed 856,380 first-time mothers without previous venous thromboembolism for 90 days after delivery, linking each to relatives through the multi-generation register. 788 (0.09%) developed postpartum VTE.

An affected grandparent was associated with HR 1.41 (95% CI 1.21 to 1.64), a parent 2.03 (1.60 to 2.56), a full sibling 2.55 (1.53 to 4.26) and a half sibling 2.28 (1.08 to 4.80). A weighted hereditary score showed a clear gradient, reaching HR 3.66 (2.06 to 6.51) in the highest category after full adjustment. Obesity, pre-eclampsia, caesarean delivery, postpartum haemorrhage with transfusion and infection were independent predictors alongside.

This is observational and the absolute risk stays low, but it gives weight to taking a structured family history rather than a single yes-or-no question. The number and closeness of affected relatives matters, and that detail can tip a borderline woman into postnatal thromboprophylaxis under existing risk-scoring guidance.

Ask about VTE in parents, siblings and grandparents at booking, not just 'any family history'
Record how many relatives were affected and how closely related they are
Combine family history with caesarean, pre-eclampsia, obesity and haemorrhage when scoring postnatal risk
Revisit the score after delivery, since several risk factors arise during labour

Why it matters

A single family-history tick box understates risk for a woman with several affected first-degree relatives.

The statistics, in plain English

A hazard ratio of 2.03 for an affected parent means about twice the rate of postpartum VTE. Because the baseline risk is about 1 in 1,100, even a tripled risk remains small in absolute terms, which is why family history adds to other factors rather than deciding prophylaxis on its own.