According to the Centers for Disease Control and Prevention, women have an increased risk of blood clots for up to 6 weeks after giving birth. But new research suggests that the risk of a blood clot remains higher than normal for a minimum of 12 weeks after delivering a baby.

The research team, led by Dr. Hooman Kamel of the Department of Neurology and the Brain and Mind Research Institute of Weill Cornell Medical College in New York, presented the study findings at the American Stroke Association's International Stroke Conference 2014.

Blood clotting, also known as coagulation, is a process that stops excessive bleeding when a blood vessel is injured. Blood cells called platelets work with proteins in the plasma of the blood to form a clot over the area of injury.

The body usually dissolves the blood clot once the injury has healed. But at times, clots can form inside the blood vessels without naturally dissolving. In this case, blood can build up behind the clots, causing swelling and pain. The clots can also block the return of blood to the heart, which can cause heart attack and stroke.

The American Society of Hematology state that women are at a higher risk of blood clots during and shortly after pregnancy. Pregnant women tend to form blood clots in the deep veins of the legs or the pelvic area, which is known as deep-vein thrombosis (DVT).

Blood clots can also lead to pulmonary embolism (PE) - a condition in which a blood clot detaches and blocks the main artery of the lungs.

Postpartum blood clot risk increased for 12 weeks after birth

For the study, the investigators analyzed data of 1,687,930 women who gave birth at a hospital in California between 2005 and 2010.

Of these women, 1,015 suffered blood clots during the 1.5 years after giving birth. Some of the women experienced stroke, heart attack, PE or DVT as a result of the clots.

Using the data, the researchers found that during weeks 0-6 after birth, the risk of a blood clot for the women was 10.8 times higher. Blood clot risk was 2.2 times higher during weeks 7-12, and 1.4 times higher during weeks 13-18.

The investigators note that the increased risk during weeks 13-18 was "non-significant" and the risk of a blood clot returned to normal by weeks 19-24.

The research team says that although fewer than 1 in 10,000 women suffer a blood clot related to pregnancy in the 6 to 12 weeks after birth, the seriousness of blood clots in this population should not be overlooked.

Dr. Kamel says:

"While rare, blood clots are a serious cause of disability and death in pregnant and postpartum women, and many members of our research team have cared for young women with these complications.

Clinicians should consider our results when caring for high-risk postpartum patients, such as those with previous clots, or postpartum patients with symptoms concerning for thrombosis."

The investigators add that women who have recently given birth should seek medical attention if they experience chest pressure or pain, swelling or pain in one leg, have difficulty breathing, develop a sudden severe headache, or have sudden loss of vision, speech, balance or strength on one side of the body.

Last year, Medical News Today reported on a study suggesting that in vitro fertilization (IVF) increases the risk of blood clots during pregnancy, while a more recent study detailed a urine test that may be able to detect blood clots.