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

This Swedish register study followed 856,380 first-time mothers with no prior venous thromboembolism for 90 days after delivery, and linked them to their relatives through the national multi-generation register. In all, 788 women (0.09%) had a postpartum VTE.

Risk rose with how close and how many affected relatives there were: a parent with VTE carried HR 2.03 (1.60–2.56), a full sibling HR 2.55 (1.53–4.26), and a grandparent HR 1.41 (1.21–1.64). On a combined hereditary score, the highest category reached HR 3.66 (2.06–6.51). Obesity, caesarean delivery, pre-eclampsia, postpartum haemorrhage with transfusion and infection were independent predictors as well.

The absolute risk stays low, so a family history alone will rarely justify prophylaxis. What it does is add weight in a scoring tool: a woman with an affected parent or sibling who also has a caesarean or a high BMI has a meaningfully stacked risk.

Ask specifically about VTE in parents, siblings and grandparents at booking — not just 'any family history of clots'.
Record first-degree relatives separately; a parent or sibling roughly doubles to triples the hazard.
Add family history to the postpartum risk assessment alongside caesarean, BMI and pre-eclampsia.
Absolute postpartum VTE risk in this cohort was about 1 in 1,100, so family history adds weight rather than deciding alone.

Why it matters

A loosely asked family history under-weights a risk factor that scales with how many and how close the affected relatives are.

The statistics, in plain English

A hazard ratio of 2 means about twice the rate over the same time. Because the baseline rate is under 0.1%, doubling it still leaves most women unaffected — relative risk is high, absolute risk is low.