Heart disease is the No. 1 killer of American women, but doctors have been using data on men's heart health to determine a woman's risk for heart attack, stroke or other problems.

A huge new study could more accurately determine the risk, according to a study in the Journal of the American Medical Association.

"We currently tend to underestimate women's risk of heart disease. We found in the new data that many women were at much higher risk than we anticipated," said Dr. Paul Ridker of Brigham and Women's Hospital.

Leslie Power was 45 years old when that risk caught up with her.

"I couldn't believe I was actually having a heart attack," she said.

Ridker's team studied blood samples from almost 25,000 women to see what risk factors predicted strokes or heart attacks 10 years later.

"We created a new risk score, something called the Reynolds Risk Score, and it turned out this does a much better job predicting accurately a woman's true risk of having a heart attack or stroke over the next 10 years of her life," he said.

The score includes traditional measures such as smoking, age, diabetes, blood pressure and cholesterol. But it now includes family history and a measure of something called CRP, which indicates inflammation of the arteries.

"What's very important to understand is that you can be a high-risk patient if your CRP is elevated, even if your cholesterol levels are low," Ridker said.

Right now, about 9 million women are considered at medium risk, but half of them could change their classifications under the new score, JAMA reported.

Women who want to evaluate their heart risk can try this online tool.