Middle-aged American women are gaining weight, especially around the waist, while their risk of stroke has increased significantly, a new study finds.

"In this study, we can't determine exact cause and effect, but it suggests there might be a relationship," said Dr. Amytis Towfighi, an assistant professor of neurology at the University of Southern California, who was expected to report the findings Wednesday at an American Stroke Association's International Stroke Conference in New Orleans.

Most stroke studies focus on older people, but the incidence of stroke in women aged 35 to 54 is twice as high as in men of the same age, Towfighi said. Using data from the National Health and Nutrition Studies done in 1988-1994 and 1999-2004, she and her colleagues looked at whether the risk of stroke in middle-aged women has increased and what the causes of such an increase might be.

The increase is real, the study found. In the earlier study, 0.6 percent of women in the age group reported strokes, but that rose to 1.8 percent in the later study. Stroke incidence among men of the same age remained stable, with an incidence of about 1 percent.

"In women, waist circumference increased significantly, as did the prevalence of obesity," Towfighi noted. "There was no difference in the percentage of women who had diabetes, were smokers or who had hypertension."

Women in the later study had an average waist circumference that was 4 centimeters wider than women in the earlier study. Average body-mass index, a measure of obesity, rose from 27.11 in the earlier study to 28.67 in the later study. And 14.8 percent of the women in the later study reported using medications to lower blood pressure, up from 8.9 percent in the earlier study. Almost 4 percent of women in the later study said they were taking medication to lower cholesterol, compared with 1.4 percent in the earlier study.

"Abdominal obesity is a known predictor of stroke in women and may be a key factor in the midlife stroke surge in women," Towfighi said in a statement.

The relationship makes sense, said Daniel T. Lackland, a professor of epidemiology at the University of South Carolina, and a spokesman for the American Stroke Association. "People have shown that obesity does make a big difference in increasing the risk of stroke and cardiovascular disease," he said.

But it's difficult to disentangle the various risk factors that circle around obesity, Lackland said. "What many have shown is that if you increase obesity, you increase the risk of high blood pressure, diabetes and also lipid abnormalities," he said, a reference to high cholesterol levels. "Is obesity an independent risk factor for stroke? The study was not designed to show that."

The study does reinforce the standing advice to avoid obesity, as well as other stroke risk factors, Lackland noted. "By losing weight, you lose abdominal circumference, you reduce the risk of diabetes and lipid abnormalities, all of which are known risk factors for cardiovascular disease," he said.

A recent national study found that uncontrolled hypertension rates are increasing among American women of all ages, with 22 percent of women having high blood pressure in the early 2000s, compared with 17 percent in the 1990s. The incidence among American men dropped from 19 percent to 17 percent during that same period, but the rate of decline among men has slowed, the study found.

Learn more: www.americanheart.org, the American Heart Association has more information about stroke and its causes.