The study by scientists in Belgium suggests the pill may increase a woman's risk of heart attack or stroke.

The researchers from Ghent University say women who have used oral contraceptives are more likely than those who had not taken the pill to have a buildup of plaque in their arteries.

Dr. Ernst Rietzschel who led the study says the main concern is that higher plaque levels increase the risk of a clot developing on one of the plaques resulting in a stroke, heart attack or sudden cardiac death.

Dr. Rietzschel and his team studied 1,301 women ages 35 to 55; of that number 81 percent had used the pill, for an average of 13 years.

The researchers found a rise of 20 to 30 percent in arterial plaque in the two big arteries, the carotid in the neck and the femoral in the leg, for each decade of use.

Plaque levels were measured using a technique called vascular echography.

Atherosclerosis results in a hardening and narrowing of the arteries caused by the slow buildup of plaque, made up of fat, cholesterol, calcium and other material, on the inside of artery walls.

Rietzschel says the findings should not trigger alarms about the safety of the pill but women should take other steps to reduce their cardiovascular disease risk such as eating a healthier diet, getting more exercise, not smoking and controlling cholesterol.

But he also says oral contraception is not the only form of contraception and women could explore other options.

Experts have expressed surprise at the results but say the finding needs to be tested in larger and more rigorous studies.

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

The birth control pill which uses hormones to suppress ovulation is now taken by 100 million women worldwide.

The research was presented at an American Heart Association meeting.

In addition, the study found people who did not carry the ApoE4 gene and consumed an unbalanced diet characterized by regular use of omega-6 rich oils, but not omega-3 rich oils or fish were twice as likely to develop dementia compared to those who didn't eat omega-6 rich oils, which include sunflower or grape seed oil. The study did not find any association between consuming corn oil, peanut oil, lard, meat or wine and lowering risk of dementia.

While we've identified dietary patterns associated with lowering a person's risk of dementia or Alzheimer's, more research is needed to better understand the mechanisms of these nutrients involved in these apparently protective foods, said Barberger-Gateau.

The study was supported by the National Agency for Research in France.

The American Academy of Neurology, an association of more than 20,000 neurologists and neuroscience professionals, is dedicated to improving patient care through education and research. A neurologist is a doctor with specialized training in diagnosing, treating and managing disorders of the brain and nervous system such as Parkinson's disease, ALS (Lou Gehrig's disease), dementia, West Nile virus, and ataxia.

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