Doctors have known for a long time that using birth control pills for three years or more can cut the risk of ovarian cancer 30% to 50%. But what they didn't know was why.

Now, a study reported in the Journal of the National Cancer Institute (Vol. 94, No. 1: 32-38) suggests that progestin, a hormone found in birth control pills, may be the explanation.

The authors, Joellen Schildkraut, PhD, and colleagues from Duke University School of Medicine, Durham, N.C., and the Centers for Disease Control and Prevention in Atlanta, knew that research showed that progestin appeared to protect against cancer in the ovaries of monkeys.

This effect, called apoptosis, is a process of cell death that helps the body remove damaged cells that could develop into cancer. If the progestin caused apoptosis to occur in the ovaries of the monkeys, then perhaps the same progestin in birth control pills could explain their beneficial effect in women.

Until now, no one had examined specific hormones and medication concentrations in birth control pills to find out if progestin was the reason that the pills decreased the risk of ovarian cancer.

The researchers looked at information that had been collected from the Cancer and Steroid Hormone Study (CASH) conducted 20 years ago.

They interviewed 579 women (aged 20 to 54) from the CASH study who were diagnosed with ovarian cancer from 1980 to 1982. They also interviewed 4,754 women who did not develop the disease.

In the interviews they asked which women had used birth control pills for three or more months, and what type of pill they were using at that time.

The results showed that women who used pills with a high progestin strength were more protected from developing ovarian cancer than women who used lower strength medications.

They also found that women who did not use birth control pills were almost three times more likely to develop ovarian cancer than women who did, whatever the concentration of estrogen or progestin in the pill.

Also, the longer a woman took the pill, the greater the benefit of reducing ovarian cancer risk.

The authors cautioned that they could not conclude whether these findings apply to menopausal women, since the women who participated in the CASH study were relatively young compared to the age of women who generally develop ovarian cancer.

The authors said their research suggests that a medicine could be developed to provide "maximum protection against ovarian cancer, while minimizing side effects."

According to the American Cancer Society (ACS), ovarian cancer is the seventh most frequent non-skin cancer in women. There will be an estimated 23,300 new cases diagnosed in the US in 2002, and 13,900 deaths.

There is no accepted effective screening test for ovarian cancer, and usually when a woman has symptoms it is a more advanced disease.

Periodic pelvic exams are an important part of a woman's health checkup, and the most common way doctors check for ovarian cancer. When found early there is a high rate of cure.

Several factors other than contraceptives are known to decrease the risk of ovarian cancer, including pregnancy and breast-feeding.

The risk of developing the disease increases with age and peaks in the late 70s, according to ACS.

Women with breast cancer, or with a family history of breast or ovarian cancer, are at increased risk of developing ovarian cancer.