The risk of stroke among pregnant women and those who have just given birth is growing at an alarming rate, say U.S. researchers.

The increase is due to women having increased risk factors for stroke before becoming pregnant, including high blood pressure and obesity, according to a study published Thursday in Stroke: Journal of the American Heart Association.

Pregnancy-related stroke hospitalizations in the United States increased 54 per cent, from 4,085 in 1994-95 to 6,293 in 2006-07. (Michaela Rehle/Reuters)

Researchers found that across the United States, pregnancy-related stroke hospitalizations increased 54 per cent, from 4,085 in 1994-95 to 6,293 in 2006-07.

Those numbers include both pregnant women and women who had given birth in the previous 12 weeks.

Given the increased number of pregnant women with pre-existing risk factors, the researchers said they were expecting to find more were being hospitalized for pregnancy-related strokes.

The magnitude of the increase, however, was a surprise, said Dr. Elena Kuklina, an epidemiologist at the Centers for Disease Control and Prevention.

"When you're relatively healthy, your stroke risk is not that high," Kuklina, lead author of the study, said in a release.

"Now more and more women entering pregnancy already have some type of risk factor for stroke, such as obesity, chronic hypertension, diabetes or congenital heart disease. Since pregnancy by itself is a risk factor, if you have one of these other stroke risk factors, it doubles the risk."

The study also found that pregnant and post partum women aged 25 to 34 were hospitalized for stroke more often than those who were younger or older. The researchers did not suggest possible explanations.

Pregnant women are especially difficult to treat because they are usually excluded from clinical trials of medication because of concern drugs can harm the fetus.

As a result, doctors don't have enough information about which medications are best for pregnant women who are at an increased risk for stroke.