Introduction: Combined oral contraceptive (OC) use strongly and consistently reduces the risk for epithelial ovarian cancer (EOC); longer durations of use and more recent use are associated with the strongest reductions in risk. However, it is unknown if exclusive OC use before the first birth is associated with a reduction in EOC risk many years later. Therefore, we investigated the risk for EOC among parous women associated with exclusive OC use before the first birth. Methods: From a population-based case-control study in Alberta and British Columbia, Canada, 2001–2011, we included 1144 invasive EOC cases and 2513 controls who were >40 years at diagnosis/reference date. Participants reported OC use and all pregnancies via a telephone interview or self-administered questionnaire (in the early years of the study). Duration of OC use was evaluated as a continuous variable and by categories: non-users (never use or <6 months of use), <5, 5–<10, >10 years, unknown. Using logistic regression, we estimated adjusted odds ratios (aORs) and 95% confidence intervals (CI), controlling for study site, age, parity, breastfeeding, first degree family history of breast/ovarian cancer, tubal ligation, and BMI. Results: OC use at any time during reproductive life was associated with a 42% reduced risk for EOC relative to non-users (aOR = 0.58, 95% CI, 0.49–0.69). Among parous women, each additional year of exclusive OC use before the first birth conferred an 11% risk reduction relative to non-users (aOR = 0.89 95% CI, 0.86–0.94, linear trend p-value <0.01). Results were similar when we restricted to cases with high grade serous cancers (aOR = 0.90 95% CI, 0.84–0.95, linear trend p-value <0.01) and for cases with endometrioid/clear cell cancer (aOR 0.88 95% CI, 0.80–0.95, linear trend p-value < 0.01). Discussion: Among parous women, exclusive use of OCs before the first birth was associated with a strong reduction in EOC risk many years later. Because OCs stop ovulation, this reduced risk may be due to a reduction in lifetime ovulatory cycles. However, it is also possible that OC use at younger ages, before the first birth, represents a window of opportunity to have a substantial impact on reducing risk that remains for many years, informing possible prevention strategies.

The following are the 17 highest-scoring abstracts of those submitted for presentation at the 40th Annual ASPO meeting held March 13–15, 2016, in Columbus, OH.