TIME Pregnancy

Increased Risk of Blood Clots Lasts 12 Weeks After Pregnancy, Not Just Six

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New research shows that women are at risk of stroke or heart attack after giving birth for longer than doctors originally thought

Women have a heightened risk of blood clots for 12 weeks following birth, twice as long as doctors originally believed, according to new research. Blood clots can cause problems such as stroke or heart attack.

The head of the study, Dr. Hooman Kamel, presented the new research, published online in the New England Journal of Medicine, on Thursday at the American Heart Association stroke conference. Women are more prone to blood clots after giving birth because blood components increase during labor to prevent too much bleeding. As a result, blood from the legs has more trouble traveling to the heart. If clots in the legs travel to the lungs, they can be fatal. Strokes are rare after pregnancy, but result in death about 10% of the time.

Doctors sometimes prescribe blood thinners to women at high risk of blood clots for six weeks after pregnancy, but the new study suggests that isn’t long enough. The study followed 1.7 million California women who gave birth to their first child. Over the next year and a half, 1,015 of them developed clots — 248 of whom had strokes and 47 of whom had heart attacks. The risk of blood clot was 11 times greater in the first six weeks and more than two times greater for the six weeks following that.

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