Patient Education Blog

Blood Clots After Delivery – How Long is a Woman at Risk?

Stephan Moll, MD writes… Women are at increased risk for blood clots (DVT, PE, stroke and heart attacks) during pregnancy and in the weeks after delivery.  If the risk is high enough, then preventive blood thinners (anticoagulants) are recommended, such as in a woman with a history of a previous blood clot. Guidelines exist as to which woman should be considered for blood thinners (ACOG 2012 guideline; ref 1).

The period after delivery is a particularly high-risk period for forming blood clots. It has typically been defined as being the 6 weeks after giving birth.  Therefore, if blood thinners are chosen after delivery, they are classically given for 6 weeks.  A study recently published in the New England Journal of Medicine (ref 2) examined whether the risk for blood clots is increased only for 6 weeks postpartum or whether the risk persists beyond the first 6 weeks.

Study Methods

The investigators analyzed data of 1,687,930 women who gave birth in a hospital in California between 2005 and 2010.  They determined how many women developed a blood clot – DVT or PE, stroke or heart attack – in the 1 ½ year following the delivery. They then analyzed when after delivery the clots occurred.  The baseline risk was considered the blood clot risk of women who were more than 1 year after delivery.

 Study Results

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

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

b)    How many women developed a clot?

    • Even though the risk for clot is increased in the time period after giving birth, the overall risk in the general population is very low: in the first 6 weeks after delivery only 1 of 4,100 women developed a clot (DVT, PE, stroke or heart attack), in week 7-12 after giving birth, only 1 in 17,800 women.

c)     What type of clots is the woman at risk for?

    • Most of the clots that occurred in the 3 months after giving birth were DVT or PE (70%); stroke was the 2nd most common clot (26% of all clots); heart attack was rare (4 %).

Study Conclusions

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


Implications of this study

The woman who is at high risk for DVT and PE in the post-partum period should discuss with her physician whether 6 weeks or 3-4 months of treatment with blood thinners is appropriate for her.  This decision is influenced by the sum of all her DVT/PE risk factors, including her personal history of clots, the presence of a strong clotting disorder or strong family history of DVT/PE, obesity, and C-section delivery.  The decision would also take into account the woman’s risk factors for bleeding and her personal preference.



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


Last updated:  March 1st, 2014

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