The postpartum state is associated with a heightened risk of thrombosis. The duration of heightened risk for postpartum CVT is uncertain.

Using claims data from the Healthcare Cost and Utilization Project (HCUP) from all emergency departments and acute care hospitalizations in Florida (2005-2015) and New York (2006-2015), we identified women aged ≥18 years old who were hospitalized for labor and delivery. For women with multiple labor-related hospitalizations during a single 40-week period, we excluded cases of false labor by identifying delivery as the latest hospitalization during that time. Patients with claims for CVT before their first recorded delivery, and women with a second delivery during the follow-up period were also excluded. We compared the likelihood of a first-ever recorded CVT during postpartum days 0-41 compared with the same period 1 year later. We repeated this crossover-cohort analysis for consecutive 6-weeks periods after delivery, as compared with the corresponding 6-week period 1 year later. We used McNemar's to calculate odds ratios for each 6-week interval.

Among the 1,406,447 women with a first recorded delivery, the risk of CVT was markedly higher within the 6 weeks postpartum than in the same period 1 year later (22 versus 3 CVTs per million deliveries). This corresponded to an absolute risk difference of 19 events per million (95% CI, 11-27) and an odds ratio of 10.0 (95% CI, 3.1-51.2). There was no significant increase in the risk of CVT during the period of 7-12 weeks after delivery as compared with the same period 1 year later with an absolute risk difference of 6.0 events (95% CI, 0-11) per million deliveries and an odds ratio of 3.0 (95% CI, 0.9-12.8).