According to research published in the British Journal of Cancer, the use of oral contraceptives and parity show a strong protective association against ovarian cancer risk.
  

It is well established that parity and use of oral contraceptives reduce the risk of ovarian cancer, but the associations with other reproductive variables are less clear. Researchers therefore evaluated the associations of oral contraceptive use and reproductive factors with ovarian cancer risk in the European Prospective Investigation into Cancer and Nutrition study. The study had involved 327,396 women, of which 878 patients had developed ovarian cancer over an average of 9 years. The following results were reported:

• Women who used oral contraceptives for 10 or more years had a 45% (HR, 0.55; 95% CI, 0.41–0.75) lower risk compared with users of 1 year or less (P-trend, <0.01).
• Compared with nulliparous women, parous women had a 29% (HR, 0.71; 95% CI, 0.59–0.87) lower risk, with an 8% reduction in risk for each additional pregnancy.
• A high age at menopause was associated with a higher risk of ovarian cancer (>52 vs ,/=45 years: HR, 1.46; 95% CI, 1.06–1.99; P-trend, 0.02).
• Age at menarche, age at first full-term pregnancy, incomplete pregnancies and breastfeeding were not associated with risk.

The researchers conclude that “this study shows a strong protective association of oral contraceptive use and parity with risk of ovarian cancer, a higher risk with a late age at menopause, and no association with other reproductive factors in this large cohort of European women.”