Ovarian and Endometrial Cancers and Oral Contraceptives

Most studies have found that OCs reduce the risk of cancers of the ovary. The Centers for Disease Control and Prevention's (CDC) Cancer and Steroid Hormone Study, along with other research conducted over the past 18 years, shows that the longer a woman used OCs, the lower her risk of ovarian cancer. Moreover, this lowered risk persists long after OC use ceases. The most recent report from the study found that the reduced risk is seen in women who have used OCs for as little as 3 to 6 months, and that it continues for 15 years after use ends. While several hypotheses have been offered to explain how OCs might protect against ovarian cancer, the exact mechanism is still not known. Authors of the report estimate that OC use averts more than 1,700 cases of ovarian cancer in the United States each year.

Researchers have also found that OC use may reduce the risk of endometrial cancer. Results of the Cancer and Steroid Hormone Study support previous reports that combination OC use can avert the development of endometrial cancer. The study found that using combination OCs for at least 1 year reduces the risk of developing endometrial cancer to half of that seen for women who never took birth control pills. Additionally, the beneficial effect of OC use appears to persist at least 15 years after OC users stop taking birth control pills. Other studies have found that taking combination OCs for less than 1 year gives no reduction in risk of endometrial cancer and that the longer the OC use, the lower the risk.

The reduction in risk of ovarian and endometrial cancers from OC use does not apply to the sequential type of pill, in which each monthly cycle contains 16 estrogen pills followed by 5 estrogen-plus-progesterone pills. (Sequential OCs were taken off the market in 1976, so few women have been exposed to them.) Researchers believe OCs reduce cancer risk only when the estrogen content of birth control pills is balanced by progestogen in the same pill. 

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