The Centers for Disease Control and Prevention's (CDC) Cancer and Steroid Hormone Study (CASH), along with other research conducted over the past 20 years, shows that the longer a woman uses OCs, the lower her risk of ovarian cancer. Moreover, this lowered risk persists long after OC use ceases. The CASH study found that the reduced risk of ovarian cancer 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. Other studies have confirmed that the reduced risk of ovarian cancer continues for at least 10 to 15 years after a woman has stopped taking OCs.
Several hypotheses have been offered to explain how oral contraceptives might protect against ovarian cancer, such as a reduction in the number of ovulations a woman has during her lifetime, but the exact mechanism is still not known.
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.
Source: National Cancer Institute (http://cis.nci.nih.gov/fact/3_13.htm)