Researchers analyzed and checked data collected from 45 epidemiological studies of ovarian cancer conducted in 21 countries, providing data from over 23,000 women with ovarian cancer, and over 87,000 without ovarian cancer. 31 percent (7,308) of the women with ovarian cancer, and 37 percent (32,717) of the women without ovarian cancer, had a history of oral contraceptive use.
The risk reduction persisted for more than 30 years after women stopped taking the pill, but decreased as time went on. The proportional risk reduction for over 5 years of use was 29 percent up to 10 years after stopping use, 19 percent for 10-19 years after stopping, and 15 percent for 20-29 years after stopping. The results were also linked to a reduction in mortality rate from 7 to 5 deaths per 1,000 women.
Lead author, Valerie Beral, claims, "Oral contraceptives have already prevented some 200,000 ovarian cancers and 100,000 deaths from the disease, and that over the next few decades the number of cancers prevented will rise to at least 30,000 per year."