ORLANDO, Fla. — Birth control pills have been linked for the first time to plaques that could potentially endanger the heart, doctors here said Tuesday.

A study of 1,301 women ages 33 to 55 suggests that the likelihood of finding plaques in key arteries increased by 20 percent to 30 percent for every 10 years of pill use, Ernst Rietzschel, of Ghent University in Belgium, told an American Heart Association meeting here.

The study also links the pill to potentially artery-clogging plaque in women who no longer use oral contraception, he says, noting that 81 percent of women in the study took the pill, on average, for 13 years. Doctors cautioned that the study is small and should be confirmed by more research.

The pill is the world's most popular form of contraception. About 12 million women take it in the United States, says the U.S. Centers for Disease Control and Prevention.

Despite the widespread use of the pill, researchers have never linked it with heart attacks or strokes, though it's well known to increase blood pressure and raise a woman's risk of getting blood clots. Many of the women in the study used pills with higher doses of estrogen than are used today. "We don't know whether effects will be the same for women taking the pill today," Rietzschel says, adding, "There's no reason to stop taking the pill abruptly; the study needs to be replicated."

"I think it's a wake-up call," says Jennifer Mieres of New York University School of Medicine. "If you're going to take birth control pills, you need to be aggressive about reducing your other risk factors: smoking, high blood pressure, cholesterol, diet and lifestyle."

The finding turned up by chance, when doctors studying residents in two Belgian towns found that women who used the pills had triple the expected levels of an enzyme linked with cardiovascular inflammation.

Researchers used ultrasound to test the carotid arteries on each side of the neck and femoral arteries in each leg. A woman's odds of having plaque in one carotid artery went up by 17 percent and in both carotids by 42 percent for every 10 years of pill use. The odds of having plaque in a single femoral artery went up by 28 percent and in both by 34 percent.

"I'm struck by this, because I'm telling patients that there's no evidence of any increased risk for a woman now because she once used birth control pills," says Sharon Hayes of the Mayo Clinic. "The fact that these investigators have found it warrants further study."