"Risk of a Thrombotic Event after the 6-Week Postpartum Period"
NEJM
http://www.nejm.org/doi/full/10.1056/NEJMoa1311485?query=featured_home 
"CONCLUSIONS

Among patients in our study, an elevated risk of thrombosis persisted until at least 12 weeks after delivery. However, the absolute increase in risk beyond 6 weeks after delivery was low. (Funded by the National Institute of Neurological Disorders and Stroke.)"