Pregnant women are more prone to blood clots because substances that thicken the blood naturally rise, and blood from the legs has more trouble traveling to the heart. Clots can cause heart attacks and strokes.
The study involved 1.7 million California women having their first child. The risk of a blood clot was about 11 times greater during the first six weeks after delivery and more than two times greater during weeks seven to 12. After that, it fell to normal levels.
The work was published online in the New England Journal of Medicine and presented at a stroke conference in San Diego on Thursday.