Breast cancer and oral contraceptives: patterns of risk among parous and nulliparous women--further analysis of the Swedish-Norwegian material.

A Swedish-Norwegian case-control study comprising 473 women less than 45 years old with newly diagnosed invasive breast cancer diagnosed in 1984-85, and 722 age-matched control women, was reanalyzed to evaluate if nulliparous women who had used oral contraceptives (OCs) were at particular risk for breast cancer. The relative risk for nulliparous women who had used OCs for eight years or more was 4.3 (95% confidence interval, 1.4-13.1), and parous women with the same duration of use had relative risk 1.7 (0.7-4.2) as compared to nulliparous and parous women, respectively, who had never used OCs. Parous women who had used OCs for twelve years or more after their first full-term pregnancy had a relative risk of 3.0 (1.3-7.4). The findings from the study suggest that nulliparous women may be particularly susceptible to the risk for breast cancer with long-term OC use.