Family history of clots carries a graded postpartum VTE risk

Design

Nationwide population-based cohort, Sweden 1992 to 2018

Population

856,380 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 family score HR 3.66 (2.06 to 6.51)

A Swedish nationwide cohort (AJOG, 7 October) followed 856,380 first-time mothers with no previous venous thromboembolism (VTE) for 90 days after delivery between 1992 and 2018, linking them to their relatives through the national multi-generation register. In all, 788 women (0.09%) had a postpartum VTE.

After adjustment, a VTE in a parent was associated with about twice the risk (HR 2.03, 95% CI 1.60 to 2.56), in a full sibling HR 2.55 (1.53 to 4.26), and in a grandparent HR 1.41 (1.21 to 1.64). A score weighting relatives by closeness showed a clear gradient: HR 1.50 for the lowest category, 2.89 for the middle and 3.66 (2.06 to 6.51) for the highest, where several close relatives were affected. Obesity, pre-eclampsia, caesarean delivery and postpartum haemorrhage with transfusion were also independent predictors.

Most postpartum risk tools score family history as present or absent. This study suggests the number and closeness of affected relatives matters, and a grandparent alone is weaker than a sibling or parent. Taking a fuller family history at booking, and counting relatives, sharpens the thromboprophylaxis decision without any new test.

• At booking, ask which relatives have had a clot, not just whether anyone has.
• Weigh a parent or sibling with VTE more heavily than a grandparent or cousin in the postnatal risk assessment.
• Where several close relatives are affected, consider that the risk may be several-fold higher than baseline.
• Reassess VTE risk after delivery: caesarean, pre-eclampsia and haemorrhage with transfusion each add risk.
• Use the local thromboprophylaxis guideline for the final decision; this study refines the history, not the threshold.

Why it matters

Family history is usually a tick-box; this shows its weight depends on who was affected and how many.

Don't overread it

This is observational; it does not test whether more thromboprophylaxis for these women prevents clots.

The statistics, in plain English

Baseline postpartum VTE was rare, about 9 in 10,000 first-time mothers. A hazard ratio of 3.66 means roughly three to four times that baseline, which is still under 1% in absolute terms. The highest category had few women, so its interval is wide (2.06 to 6.51).

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