Education Blog for Healthcare Professionals

How Long is the Post-Partum Period in Respect to Thrombosis Risk?

Stephan Moll, MD writes… A recent NEJM study (ref 1) examined whether the risk for thrombosis in women persists beyond the first 6 weeks after delivery.  It found that an increased risk persists for at least 3 months after delivery, although the absolute risk was low after the first 6 weeks.  This is of clinical relevance, as the post-partum period has traditionally often been defined as the 6 weeks after delivery and, if post-partum anticoagulation prophylaxis is considered, it is typically given for 6 weeks only (ACOG – ref 2).


The investigators analyzed data of 1,687,930 women who gave birth in a hospital in California between 2005 and 2010 and determined how many women developed a thrombotic event – DVT or PE, stroke or heart attack – in the 1 ½ year following the delivery. The baseline thrombosis risk was considered the thrombosis risk > 1 year after delivery.

 Study Results

a)    When is the high-risk period for thrombosis?

    • The results are presented in Figure 1 (modified for clarity from NEJM publication)
      • During week 0 to 3 after birth: The vast majority of thrombotic events occurred in the first 3 weeks after delivery. In this time period the risk for thrombosis was 18.3 times higher than a woman’s normal baseline risk.
      • During week 4-6 after birth, the risk had lessened, but was still moderately increased (4.1 times higher than baseline).
      • From 1½ to 4 months (week 7-15) after delivery the risk was only slightly increased, being about 2.0 times higher than baseline.
      • By 4 months after delivery the risk was back down to a woman’s normal low baseline risk of clots.

b)    What is the absolute risk to develop a thrombotic event?

    • Even though the risk for clot was increased in post-partum period, particularly in the first 3 weeks, the overall risk in this general population group of women was very low: in the first 6 postpartum weeks only 1 of 4,100 women developed a thrombotic event; in week 7-12 only 1 of 17,800 women.

c)     What type of thrombotic event is a woman at risk for?

    • Of the thrombotic events that occurred in the 3 months postpartum, 70 % were DVT or PE, 26% were stroke, and 4 % MIs.
    • The postpartum state particularly leads to an increased risk of VTE (figure 2).

Study Conclusions

The risk for thrombosis (DVT, PE, stroke and MIs) is highest in the 3 weeks after delivery, but some increased risk persists for up to 4 months after delivery.

Implications of this study for clinical practice

In women who are at high risk for pregnancy-associated VTE, an individual assessment should be undertaken whether the woman should be treated with 6 weeks of postpartum blood thinners, or whether 3-4 months might be appropriate. This decision will depend on a woman’s conglomerate  VTE risk factors, her bleeding risk, and her personal preference.


  1. Kamel H et al.  Risk of a Thrombotic Event after the 6-Week Postpartum Period. N Engl J Med, Feb 13,2014.
  2. Thromboembolism in Pregnancy. Obstetrics & Gynecology. 118(3):718-729, September 2011.  Practice bulletin # 123.
Last upated: March 1st, 2013

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