Her workday at a local school had just ended when 54-year-old Joanne Puopolo developed a headache so severe she had to pull her car off the road. The Duxbury, Mass., teacher's aide had been feeling sick all day and figured she'd caught "a bug" from one of her students. But soon after walking into her house, her left arm went stiff, and then her left hand suddenly flew up in the air. Her husband called 911, and by the time the ambulance arrived, Puopolo's entire left side was paralyzed. "I knew something was really wrong, but I wasn't thinking it could be a stroke," Puopolo says. After all, strokes only happen to elderly people, don't they? And men are much more vulnerable than women, right?

While the risk of stroke is still comparatively small before age 65, new trend data suggest that age and gender rates are starting to tip in the wrong direction for women. Not only are older women now as likely as older men to suffer a stroke, but recent health surveys indicate that the proportion of women between the ages of 35 and 54 having strokes has nearly tripled in the last decade, while the rate for men the same age has remained flat. While it's always possible that the number of strokes could have risen because of more accurate diagnoses, the fact that the same self-reporting survey was used repeatedly in a relatively brief period, gives experts confidence that the rise is real.

The new incidence rates are still small—only 1.8 percent of women between the ages of 35 and 54 report having had a stroke. (About 80 percent of "brain attacks" are ischemic, caused by a blood clot or plaque that cuts off blood flow to the brain. The remainder are hemorrhagic and occur when a blood vessel in the brain leaks or ruptures.) But stroke researcher Amytis Towfighi, an assistant professor of neurology at the University of Southern California, was worried by the new numbers, a big increase from the .63 percent of women reporting strokes in this age group a decade ago. Towfighi also knew that researchers had actually expected incidence rates to fall, as the number of people taking medication to control their blood pressure and cholesterol levels rose.

This puzzling turn of events warranted more study, Towfighi decided. "I wanted to understand if this was a new phenomenon, and if so, why it was happening," she says. "Most [stroke] studies focus on individuals who are over 60. But there are now more than 1 million stroke survivors aged 25 to 59 in the United States."

After scrutinizing data from the National Health and Nutrition Surveys conducted between 1988 and 1994 and again between 1999 and 2004, Towfighi and her former colleagues at the UCLA Stroke Center were able to publish a series of studies. They confirmed that the spike in strokes among middle-aged women is new and real, and they were also able to hypothesize about the cause of the increase: the expanding waistlines and rising body mass index (or BMI, a measure of body fat based on weight and height) of midlife women.

"There was really no significant difference in women's risk factors like smoking, high blood pressure, cholesterol levels and diabetes," Towfighi says. "But when we looked at biomarkers we found that these women's BMIs and waist circumferences had increased significantly." Men had also gained weight and inches, but not as much as women. (Overall, 50 percent more women had moved into the worrisome zone with waist circumferences over 35 inches, while only one-third more men had passed 40 inches, the outer limit of normal for males.) This was a disturbing trend, Towfighi said, because abdominal fat increases the risk of stroke more than fat deposits in other places because it's "visceral" fat, meaning it wraps itself around vital internal organs.