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Summary: Previous studies have shown that taking birth control pills reduces ovarian cancer risk. A comprehensive study published in The Lancet shows just how much.

Oxford University Professor Valerie Beral, along with researchers from the Collaborative Group on Epidemiological Studies of Ovarian Cancer, which includes researchers at the American Cancer Society, analyzed data from 45 studies conducted in the 1960s, '70s, and '80s. They found that the longer a woman took birth control pills, the lower her risk of ovarian cancer.

Why It's Important: Ovarian cancer typically strikes late in life (half of all ovarian cancer cases are found in women over 63), and prognosis is often poor, largely because the disease is usually caught in its later stages. Ovarian cancer symptoms, which include abdominal swelling and digestive problems, are easily confused with other disorders, and there is currently no good screening test available to women.

According to the study, if the current level of oral contraceptive use remains steady, 30,000 cases of ovarian cancer worldwide could be prevented each year. Even better, the protection that comes with the pill seems to start within a year of taking it and increase over time.

What's Already Known: Scientists have known for some time that birth control pills protect against ovarian cancer, and that certain lifestyle factors may affect risk. Age, weight, and a woman's reproductive and family history also appear to play a role in the likelihood a woman might develop ovarian cancer.

However, there's also some data that suggests oral contraceptive use increases the risk of other cancers. Cervical cancer has been linked to long-term birth control pill use, as has breast cancer. (For more information, see "What are the Risk Factors for Cervical Cancer?" and "What are the Risk Factors for Breast Cancer?")

But the picture is complicated. "Oral contraceptives slightly and temporarily increase the risk of breast and cervical cancer. However, the slight increase in breast cancer occurs during and in the 5 years following the use of these contraceptives, and usually at a time and age when risk is very low," said Debbie Saslow, PhD, director of breast and gynecological cancers at the American Cancer Society.

How This Study Was Done: Researchers mined data from 45 studies that included 23,257 women with ovarian cancer and 87,303 without the disease. They determined whether the women had taken birth control pills and for how long.

Scientists then examined the relationship between oral contraceptive use and ovarian cancer diagnosis during the 1960s, 1970s, and 1980s. The amount of estrogen used in birth control pills lessened over the years, and they wanted to see if different estrogen doses had different effects on ovarian cancer risk. They also adjusted for the role of age, ethnicity, education, reproductive history, family cancer history, use of hormone replacement therapy, body mass index, and consumption of tobacco or alcohol, in addition to other factors.

What Was Found: Of the women with ovarian cancer, 31% had taken the pill at some point in their lives, compared to 37% of the women who did not have ovarian cancer. The longer women used the pill, the lower their ovarian cancer risk. Taking the pill for 15 years or more cut a woman's risk of ovarian cancer by 58%; 10-14 years of pill use cut risk by 44%; and 5-9 years of use cut risk by 36%. But even women who used the pill for only 1-4 years saw a benefit; their risk was cut by 22%.

Although the benefit of the pill got weaker the longer it had been since women took it, the protective effect was still significant even 30 or more years after pill use stopped. The researchers did not see a different level of risk reduction from different estrogen doses in the pill.

"It is reasonable to expect that even current lower dose pills will be as protective as the older versions. However, we do not have definitive evidence to show this and won't for many years," said Saslow.

The study authors estimated that during the past 50 years, 200,000 cases of ovarian cancer and 100,000 deaths worldwide have been prevented by oral contraceptive use, and that if use remains at the current level, as many as 30,000 ovarian cancers could be prevented a year.

The Bottom Line: In an editorial accompanying the study, Eduardo L. Franco and Eliane Duarte-Franco of the Departments of Oncology and Epidemiology at McGill University call this study a "major contribution to our understanding of the role of oral contraceptives in the causation or prevention of ovarian cancer." But they caution that calculating the overall effect on women's health will be tricky.

While birth control pills could reduce a woman's risk of developing ovarian cancer by close to 50%, they could also raise the risk for cancers of the breast and cervix. And though these drugs are generally considered safe, they can also cause side effects like blood clots. Birth control pill use should be evaluated on a case-by-case-basis, they say.

The American Cancer Society does not currently make recommendations about taking oral contraceptives as a prevention measure against ovarian cancer, Saslow said.

And while this report is good news in the fight against ovarian cancer, there's a lot more that needs to be done. Only 20% of ovarian cancers are found at an early stage, and just 45% of women who receive an ovarian cancer diagnosis live past 5 years. More research focused on finding ovarian cancer early and improving treatment is needed, as well as on the best management and care for women who see a doctor because they have symptoms that may be associated with ovarian cancer.

For more information, see our detailed guide, Ovarian Cancer.

Citation: "Ovarian cancer and oral contraceptives: collaborative reanalysis of data from 45 epidemiological studies including 23,257 women with ovarian cancer and 87,303 controls." Published in the January 26, 2008 The Lancet (Vol. 371:303-314). Collaborative Group on Epidemiological Studies of Ovarian Cancer.

"Ovarian cancer and oral contraceptives." Published in the January 26, 2008 The Lancet (Vol. 371:277-278). Authors: Eduardo L. Franco, Eliane Duarte-Franco. Departments of Oncology and Epidemiology, McGill University, Montreal, Quebec, Canada and the Institut National de Sante Publique du Quebec, Montreal, Quebec, Canada.