Postpartum Thrombosis Risk Persists at Least 12 Weeks

February 14, 2014

The increased risk for thrombosis after childbirth persists until at least 12 weeks after delivery, twice as long as previously recognized, a new study has shown. However, the absolute increase in risk beyond 6 weeks after delivery was low.

The study was presented at the International Stroke Conference (ISC) 2014 by Hooman Kamel, MD, Weill Cornell Medical College, New York, New York, and published online in the New England Journal of Medicine to coincide with the presentation.

"While rare, blood clots are a serious cause of disability and death in pregnant and postpartum women," Dr. Kamel said. "Clinicians need to be aware that the risk of thrombosis lasts longer than just 6 weeks, especially when caring for high-risk postpartum patients, such as those with previous clots, or postpartum patients with symptoms concerning for thrombosis.

"In addition, women need to be advised to seek medical attention if they have recently delivered a baby and develop symptoms such as chest pain or pressure; difficulty breathing; swelling or pain in one leg; sudden severe headache; or sudden loss of speech, vision, balance, or strength on one side of the body."

"To our knowledge, previous studies have not reliably determined the relative risk of thrombosis beyond 6 weeks after delivery," the researchers write in the paper.

They refrain, however, from making any recommendations on antithrombotic treatment.

"Current guidelines advise that high-risk patients receive prophylactic anticoagulant therapy until 6 weeks after delivery…. Our findings suggest that the risks and benefits of continuing treatment for high-risk women beyond 6 weeks after delivery should be investigated," they write.

Dr. Hooman Kamel

Risk for Thrombosis

For the study, the researchers analyzed administrative claims data for 1,687,930 women undergoing childbirth in California from 2005 to 2010. Of those, 1015 women had a thrombotic event during the following 1.5 years.

Most of these events were venous thromboembolism (n = 720), but there were also 248 strokes and 47 myocardial infarctions. Each woman acted as her own control, by using data on thrombosis at the same time points 1 year later, thereby minimizing unmeasured confounding.

Results showed that the risk for thrombosis was increased by 10-fold in the first 6 weeks after childbirth; however, it remained increased in the 7- to 12-week period, but only by 2-fold. "The risk appears to return to normal sometime in the 13- to 18-week period after delivery," Dr. Kamel said.

Table. Rate of Thrombosis in Women Having Recently Given Birth (Cases) vs 1 Year Later (Controls)

Time After Delivery Cases (%) Controls (%) OR (95% CI)
0 - 6 wk 24.4 2.3 10.8 (7.8 - 15.1)
7 - 12 wk 5.6 2.6 2.2 (1.5 - 3.1)
13 - 18 wk 3.3 2.3 1.4 (0.9 - 2.1)
19 - 24 wk 3.1 3.1 1.0 (0.7 - 1.4)

CI = confidence interval; OR = odds ratio.


The researchers note that compared with the absolute increase in risk during the period within 6 weeks after delivery (22.1 cases per 100,000 deliveries), the absolute increase during the postpartum period of 7 to 12 weeks was much smaller (3.0 cases per 100,000 deliveries).

"During the latter period, odds ratios for thrombosis were similar for women with recognized risk factors for thrombosis and those without those risk factors, so the increased relative risk would be expected to be especially important among high-risk patients (e.g., those with an inherited primary hypercoagulable state or previous thrombosis)," they write.

The study was supported by a grant from the National Institute of Neurological Disorders and Stroke of the National Institutes of Health. Disclosure forms provided by the authors are available with the full text of the article at

N Engl J Med. Published online February 13, 2014. Abstract

International Stroke Conference (ISC) 2014. Abstract 216. Presented February 13, 2014.


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