A troubling study from Belgium hints that long-term use of oral contraceptives — at least the high-estrogen ones sold decades ago — might increase the chances of having artery buildups that can raise the risk of heart disease.

The theory needs much more rigorous testing than this single small study, but is important because of the sheer number of women now taking the pill — 100 million worldwide.

"I don't think this should be a cause for alarm among women," because many previous studies have found no large increase in heart attacks among pill users, said Dr. JoAnn Manson, chief of preventive medicine at Harvard-affiliated Brigham and Women's Hospital in Boston.

She had no role in the new study, which was presented Tuesday at an American Heart Association conference in Florida.

About 16 million American women currently take birth control pills and hundreds of millions have used them since they were first sold in 1960. Most combine synthetic estrogen and progestin in various doses.

They already are known to carry a small risk of blood clots and high blood pressure for women currently taking them, and any additional heart attack or stroke risk is thought to be related to those two effects, Manson said.

Researchers at the University of Ghent in Belgium decided to look for other signs of heart risks among past and current pill users. They studied about 1,300 healthy women ages 35 to 55 taking part in a long-running observational study in the small town of Erpe-Mere.

About 81 percent had taken oral contraceptives for more than a year at some point in their lives — similar to the prevalence the federal Centers for Disease Control and Prevention reports for American women ages 15 to 44, said the study's leader, Dr. Ernst Rietzschel. About 27 percent were current users.

Ultrasound exams were done of arteries in their necks and legs to look for buildups called plaque. There was a 20 to 30 percent increased prevalence of plaque for every 10 years of oral contraceptive use.

"These are small plaques," not nearly large enough to block an artery, Rietzschel said.

However, any plaque is thought to raise the risk of heart disease.

Researchers concede they have no information on whether the presence of plaque translated to any true risk of heart attacks or strokes in the group.

Many in the study had taken first-generation birth control pills, which had twice the estrogen levels in most ones sold today.

Doses and ages of pill use could matter, but the Belgium researchers do not have enough detail on this to tell, said Dr. Daniel Jones, a University of Mississippi cardiologist and president of the Heart Association.

"It could be an important study," but the finding needs to be tested in larger and more rigorous studies in which one group of women are given pills, another are not, and their health is closely watched for some time afterward, he said.

Rietzschel agreed, but said: "It's incredible that a drug which has been taken by 80 percent of women ... is almost bereft of any long-term outcome data, safety data."

Women worried about heart risks should not abandon birth control pills but should follow guidelines for their use and avoid other things that raise heart risks, like smoking, being overweight, and not exercising, Rietzschel said.