THURSDAY, Feb. 13, 2014 (HealthDay News) -- The increased risk of having a stroke or other blood-clotting problem might continue longer after a woman gives birth than previously believed, according to a new study.
"Historically, six weeks was the accepted period," said study researcher Dr. Hooman Kamel.
"We found the risk of blood clots remained higher than normal for twice as long as previously thought. After 12 weeks, it was no longer significant," said Kamel, an assistant professor in the Weill Cornell Medical College department of neurology and the Feil Family Brain and Mind Research Institute.
Even though the elevated risk continues for longer than previously believed, Kamel stressed that the overall risk is still low.
The study was published online Feb. 13 in the New England Journal of Medicine. Kamel, who is also a neurologist at NewYork-Presbyterian Hospital, is presenting the study Thursday at an American Stroke Association meeting in San Diego.
Kamel and his team looked at medical information from nearly 1.7 million women in California giving birth to their first child. The women delivered between early 2005 and mid-2010. Through the first year and a half after delivery, about 1,000 of the women had clotting problems, including about 250 strokes, 50 heart attacks and more than 700 cases of venous thromboembolisms (clots in the legs or lungs).
"Just like prior studies, we found the risk of these types of blood clots during the first six weeks after birth was [more than] 10 times higher than normal," Kamel said.
In the seven to 12 weeks after delivery, the risk dropped to twice the normal rate, he said. After 12 weeks, the risk was the same as when the woman was not pregnant.
Doctors monitor women for blood clots during pregnancy because the blood clots more easily at that time, according to the American Society of Hematology.
The risk of clotting problems during pregnancy is increased in women who are obese or genetically predisposed to clots, according to the society. The risk also increases in women who are inactive, on prolonged bed rest or take long trips by plane or car.
Kamel said smokers and women with chronic high blood pressure are also at increased risk.
Doctors give high-risk women low-dose blood thinners for six weeks after delivery to reduce their risk, Kamel said. Future research should focus on whether 12 weeks would be a better timeframe for the drugs, he said.
The study was funded by the U.S. National Institute of Neurological Disorders and Stroke.
The findings suggest that both doctors and pregnant women should have a heightened awareness of the potential blood clot risk longer than the traditional six-week period, said Dr. Burton Rochelson, chief of maternal fetal medicine at North Shore LIJ Health System in Manhasset, N.Y.
Rochelson reviewed the findings but did not participate in the study. He, too, emphasized that although the increased risk was found to persist, the absolute risk is small. "Even though the risk increases in that seven to 12 weeks, you are still talking about very few people relative to the total number," he said.
Since the events are serious, however, "even a small number is undesirable," he said.
One study limitation, Rochelson said, is that the researchers combined all the blood-clotting problems. In future studies, he said, researchers should consider looking separately at the risks of heart attacks, strokes and blood clots in the legs and lungs.
Women should be aware of the warning signs of a blood clot problem, Kamel said. These include chest pain or pressure; difficulty breathing; pain or swelling in one leg; sudden and severe headache; or a sudden change in consciousness, speech, balance, strength or sensation on one side of the body.
To learn more about blood clots and pregnancy, visit the American Society of Hematology.
SOURCES: Hooman Kamel, M.D., assistant professor, Weill Cornell Medical College department of neurology and Feil Family Brain and Mind Research Institute, and neurologist, NewYork-Presbyterian Hospital, New York City; Burton Rochelson, M.D., chief of maternal fetal medicine, North Shore LIJ Health System, Manhasset, N.Y.; Feb. 13, 2014, presentation, American Stroke Association meeting, San Diego; Feb. 13, 2014, New England Journal of Medicine, online