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New evidence revealed that a woman's risk of a blood clot after giving birth remains high for a longer period of time than previously presumed. According to the new study, risk of a blood clot is higher than usual for at least 12 weeks after a delivery, which is twice as long as researchers and doctors had believed.

Before conducting this study, the researchers already knew that a woman's blood clot risk is elevated during pregnancy due to platelets and other blood-clotting factors. This risk is particularly high at the time of birth. For this study, the team wanted to examine the risk after birth. They used data on 1,687,930 pregnant women who gave birth at a California hospital from 2005 to 2010. The researchers reported that 1,015 women suffered from a clot during the follow-up time span of 1.5 years. These women suffered from clots that caused stroke, heart attack, pulmonary embolism or deep vein thrombosis.

The researchers found that during the first six weeks after delivery, a woman's stroke risk increased by 10.8 times. After week six, stroke risk was 2.2 times higher than normal from weeks seven through to 12. By weeks 13 to 18, stroke risk was 1.4 times higher than normal. However, the researchers stated that this increase is a non-significant one. Stroke risk returned to normal by weeks 19 to 24.

"While rare, blood clots are a serious cause of disability and death in pregnant and post-partum women, and many members of our research team have cared for young women with these complications," said Hooman Kamel, M.D., lead researcher and assistant professor in the Department of Neurology and the Brain and Mind Research Institute of Weill Cornell Medical College in New York City. "If you have recently delivered a baby, seek medical attention if you develop symptoms such as: chest pain or pressure; difficulty breathing; swelling or pain in one leg; sudden severe headache; or sudden loss of speech, vision, balance, or strength on one side of your body."

The study was presented at the American Stroke Association's International Stroke Conference 2014.