Women have a higher risk of blood clots that can cause strokes, heart attacks and other problems for 12 weeks after childbirth — twice as long as doctors have thought, new research finds.

Strokes are still fairly rare right after pregnancy but devastating when they do occur and fatal about 10 percent of the time, according to Dr. Hooman Kamel, a neurology specialist at New York’s Weill Cornell Medical College. Blood clots in the legs usually just cause pain but can be fatal if they travel to the lungs.

Kamel led the new study, which was published online in the New England Journal of Medicine and presented at an American Heart Association stroke conference in San Diego on Thursday.

Pregnant women are more prone to blood clots because blood components to prevent excessive bleeding during labor naturally increase, and blood from the legs has more trouble traveling to the heart.

“Sometimes there’s the notion that once they deliver they don’t have to worry about these things,” but risk persists for some time after the birth, said Dr. Andrew Stemer, a Georgetown University neurologist.

Doctors now sometimes give low-dose blood thinners to certain women at higher risk of blood clots for six weeks after delivery. The new study suggests risk lasts longer than that.

It involved nearly 1.7 million California women giving birth to their first child. Over the next year and a half, 1,015 of them developed clots — 248 had strokes, 47 had heart attacks and 720 had clots in the legs or lungs.

The risk of one of these problems was about 11 times greater during the first six weeks after delivery and more than two times greater during weeks seven to 12. After that, it fell to level seen in women who had not had a baby.

A federal grant paid for the research.

Kamel advises women who recently had a baby to seek medical help right away if they develop chest pain or pressure, trouble breathing, swelling or pain in one leg, a sudden severe headache or sudden loss of speech, vision, balance, or strength on one side of the body. High blood pressure and smoking add to the risk of blood clots.

Last week, the Heart Association issued its first guidelines for preventing strokes in women, focusing on pregnancy as one high-risk time. Women with a history of high blood pressure before pregnancy should be considered for low-dose aspirin (around 81 milligrams) after the first three months of pregnancy, and calcium supplements anytime, the guidelines say.

Pregnant women with very high blood pressure (160 over 110 and above) should be treated with medications, and treatment may be considered for those with moderately high blood pressure (150 to 159 over 100 to 109). Certain blood pressure medicines are not safe during pregnancy, the guidelines note.

(AP)