Impact of established risk factors for breast cancer in nulligravid Japanese women

The mechanism by which pregnancy impacts breast cancer risk remains poorly understood. There is a need for detailed quantification of risk in nulliparous women. We therefore have undertaken a case-referent study of breast cancer employing data from the Hospital-based Epidemiologic Research Program at Aichi Cancer Center (HERPACC), Japan, examining the impact of reproductive and anthropometric factors on breast cancer risk among nulligravid women compared with their parous counterparts. In total, 2,032 breast cancer cases were included, and 17,848 women, confirmed as free of cancer, were recruited as a reference group. Odds ratios (OR) and 95% confidence intervals (95% CI) were determined by multiple logistic regression analysis. A protective effect of later age at menarche was observed among parous women, but it did not alter risk in nulligravid cases. The risk increment with a family history appeared to be most pronounced among premenopausal cases with no history of pregnancy (OR=2.68, 95% CI: 1.41-5.11). Among postmenopausal women, positive associations with height and current body mass index (BMI) in the nulligravid group were similar to those observed in the parous group. The present study indicated that age at menopause, family history in premenopausal women, and height and obesity in postmenopausal women seemed to exert more influence in nulligravid women. Formal tests for interaction between maternity status and these factors, however, did not prove statistically significant. Our findings suggest that established risk factors for breast cancer have an additive impact with nulligravid status. Thus, it is implied that obesity control for all women, including nulliparous individuals, is important from a practical viewpoint for primary breast cancer prevention.