Pregnancy increases the risk of thrombosis, and the 6-week postpartum period carries increased risks of stroke, myocardial infarction (MI), and venous thromboembolism (VTE). Whether these risks remain increased after the 6-week postpartum period is uncertain. This retrospective crossover-cohort study was performed to assess the duration of an increased postpartum thrombotic risk in a large population-based cohort of women.
Using administrative claims data on all discharges from nonfederal emergency departments and acute care hospitals in California (2005–2011), each patient’s likelihood of a first thrombotic event during sequential 6-week periods (days 42–83, 84–125, and 126–167) after delivery was compared with the corresponding 6-week period 1 year later. The primary outcome was a composite of ischemic stroke, acute MI, or VTE. Conditional logistic regression was used to calculate odds ratios (ORs) for each interval. A separate post hoc case-control analysis was done to confirm whether any increase in postpartum thrombosis was associated with labor and delivery specifically rather than with hospitalization.
Of 1,687,930 women with a first recorded hospitalization for labor and delivery during the study period, 1015 had a thrombotic event (248 with stroke, 47 with MI, and 720 with VTE) in the 1 year plus 24 weeks or less after delivery. Significantly more thrombotic events occurred within 6 weeks after delivery (411 events or 24.4 events/100,000 deliveries) than during the same period 1 year later (38 events or 2.3 events/100,000 deliveries). The absolute risk difference was 22.1 (95% confidence interval [CI], 19.6–24.6) per 100,000 deliveries, and the OR was 10.8 (95% CI, 7.8–15.1). In the period of 7 to 12 weeks after delivery, the number of thrombotic events was modestly higher compared with the same period 1 year later (95 events or 5.6 events/100,000 deliveries vs 44 events or 2.6 events/100,000 deliveries), corresponding to an absolute risk difference of 3.0 (95% CI, 1.6–4.5) per 100,000 deliveries and an OR of 2.2 (95% CI, 1.5–3.1). The risk was no longer significantly elevated for 13 to 18 week (OR, 1.4; 95% CI, 0.9–2.1) or 19 to 24 weeks postpartum (OR, 1.0; 95% CI, 0.7–1.4). A case-crossover analysis of the likelihood of labor and delivery before a first thrombotic event versus the same periods 1 year earlier found that the odds of a first delivery were markedly elevated in the period of 0 to 6 weeks before a thrombotic event (OR, 9.8; 95% CI, 7.0–13.9), significantly elevated at 7 to 12 weeks before a thrombotic event (OR, 2.2; 95% CI, 1.5–3.2), and not significantly different at 13 to 18 weeks or 19 to 24 weeks before a thrombotic event. In a separate case-control analysis, women with a thrombotic event were more likely to have been hospitalized for labor and delivery within the previous 7 to 12 weeks than to have been hospitalized for another diagnosis (OR, 1.9; 95% CI, 1.4–2.5).
The risk of a thrombotic event remained elevated beyond the 6-week postpartum period compared with a similar time period 1 year later, although absolute risk increases were small after 6 weeks. The findings are consistent with a biologic tapering of risk through at least 12 weeks after delivery, based on most coagulation markers normalizing by 6 weeks after delivery. The risks and benefits of continuing treatment for high-risk women beyond 6 weeks after delivery need additional investigations.
Department of Neurology (H.K., B.B.N., N.S., D.A.H.), Feil Family Brain and Mind Research Institute (H.K., B.B.N.), and Division of Cardiology (R.B.D.), Weill Cornell Medical College; Department of Neurology, Columbia College of Physicians and Surgeons (M.S.V.E.); and Department of Epidemiology, Mailman School of Public Health, Columbia University (M.S.V.E.), New York, NY