Protection was not significant in women who used oral contraceptives only for a short period of time, or less than 1 year during their teen years.
Researchers are still trying to determine the mechanism for the protective influence.
For each year that a woman takes an oral contraceptive her risk of ovarian cancer is reduced by about 5 percent on average, according to a study in the journal Epidemiology.The study finds among women who used oral contraceptives for more than one year the protective effect lasted 2 to 3 decades after stopping.