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.