Doctors have previously thought that women are at high risk of blood clot-related problems such as stroke and heart attack within six weeks after giving birth, but a new study has found that risks of pregnancy-related blood clot continue twice longer than previously believed.

The new study, which was presented at the American Stroke Association's International Stroke Conference Feb. 13 and published online in the New England Journal of Medicine, involved 1.6 million women who were hospitalized for labor and delivery at California hospitals from January 1, 2005 to June 30, 2010.

The researchers found that within the first year and a half of delivery, about 1,000 of the subjects had clotting problems with reported cases of 250 strokes, 50 heart attacks and over 700 venous thromboembolisms i.e. blood clots that form within veins.

"The risk of developing a clot is much higher than usual during the first six weeks after delivery, and we thought it was unlikely that the risk would suddenly return to normal at the end of six weeks, which is why we looked at what happens in the weeks following," said study researcher Hooman Kamel, an assistant professor in Weill Cornell Medical College 's department of neurology and Feil Family Brain and Mind Research Institute.

Kamel and his colleagues also observed that the risk of blood clot is at its peak around the time of delivery. The risk, however, remains about 11 times higher than normal within six weeks of giving birth, 2.2 times higher than normal from seven to 12 weeks after delivery and then decreases to almost normal after 12 weeks.

Notably, women who are obese or are genetically predisposed to blood clots have increased risks of pregnancy-related blood clots and so are inactive women, smokers and women with chronic high blood pressure.

The researchers have encouraged doctors to inform their pregnant patients of the risks. "When these events happen, there can be serious consequences, including long-term disability and death," Kamel said. "It is important to know some women will continue to be at risk, and to search for ways that they can be effectively treated."

An increase in the hormone estrogen contributes to the heightened risks of blood clots during pregnancy. Pregnant women's blood is also likely to clot to prevent excessive blood loss during childbirth. Kamel recommends that women seek medical attention if they experience sudden chest pain, swelling or pain in the leg, loss of speech and difficulty breathing as these may be symptoms of a blood clot-related problem.