Does pregnancy cause blood clots?

Pregnancy does not cause blood clots, but pregnancy does increase a woman’s chance of developing a blood clot by about fourfold. A woman’s risk is even higher immediately after delivery: in the first six weeks after delivery, a new mother’s chance of developing a blood clot is five times higher than during her pregnancy. This tendency to form blood clots post delivery likely evolved to protect women from hemorrhage at the time of miscarriage or childbirth. Nonetheless, the chance that a young, healthy woman will develop a blood clot during pregnancy is still low, about 1 to 2 in 1,000.

Eighty percent of blood clots that arise during pregnancy occur in the veins. Four of every five venous clots involves DVT, and one of every five involves PE. The other 20 percent of blood clots occur in arteries. Five of every six arterial blood clots leads to stroke, and one of every six causes a heart attack.

Women who have had a blood clot in the past have a 1 to 2 percent risk of developing a blood clot each year. This risk increases by fourfold during pregnancy, so these women’s risk of developing a blood clot during pregnancy is 5 to 10 percent. Use of anticoagulants can reduce this risk to about 1 percent. Therefore, most women who have experienced a blood clot in the past, even if they are not on anticoagulants prior to pregnancy, will be prescribed anticoagulants during pregnancy and for the six weeks after delivery. While women who had a blood clot in the past do not necessarily need to avoid pregnancy, they should be conscious of the risks to themselves and their unborn babies, consult knowledgeable physicians, and plan accordingly.