LOS ANGELES (Reuters) - The anti-cancer effect of birth control pills is becoming apparent as female baby boomers reach the age when ovarian and endometrial cancers typically show up, according to researchers.

"Starting this year, the incidence of ovarian cancer in the United States is expected to drop by 30%," Dr. Harriet Smith, of the University of New Mexico School of Medicine in Albuquerque, said at a meeting here this week of the American College of Obstetricians and Gynecologists.

That's because women who began taking oral contraceptives when the Pill became available in the 1960s are reaching age 50 or older, she noted.

Studies suggest that routine use of birth control pills, which contain the female hormones estrogen and progestin, reduces a woman's risk of ovarian cancer by 30% to 50%, said Dr. Luis Padilla-Paz, also from the University of New Mexico.

Ovarian cancer is the leading cause of gynecological cancer death in American women. Every year, some 23,000 new cases are diagnosed and 14,000 women die from the disease.

A woman's overall risk of ovarian cancer is relatively low at 1.8%, but prevalence of the disease increases with age from about 20 per 100,000 for women between the ages of 30 and 50, to 40 per 100,000 for women aged 50 to 75. Ovarian cancer most commonly occurs after menopause.

The more years of ovulation, the higher the risk of ovarian cancer. As a result, women who ovulate less because of pregnancies, birth control pills, or breast-feeding are less likely to develop the disease.

Researchers theorize that the progestin component in the birth control pill accounts for its cancer-fighting impact, rather than estrogen. Extra estrogen, a component of hormone replacement therapy sometimes prescribed to treat symptoms of menopause, has been associated with higher rates of some cancers.

Smith said the cancer-protective effect of birth control pills has been shown to persist for at least several years after a woman has stopped taking them.

Studies have also shown that oral contraceptives reduce risk of endometrial cancer, or cancer of the membrane that lines the uterus, by 50%, with each year of use cutting the risk by 11%, Padilla-Paz said.

One study showed that continuous use of hormone replacement therapy cut risk of the disease by 70%, he added.

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