Menstrual risk factors and early-onset breast cancer

Objectives: Epidemiologic studies provide evidence for increased breast cancer risk among women with prolonged exposure to endogenous estrogens and progesterone. Menstrual cycle characteristics, such as early menarche, rapid initiation of regular ovulatory cycles, short cycle length, and more days of flow, all potentially contribute to higher cumulative ovarian hormone exposure. Methods: We assessed the associations between these characteristics and breast cancer risk in a population-based, case-control study of 1505 controls and 1647 newly diagnosed cases, all younger than 45 years of age. Results: Compared to women with menarche at ≥ 15 years, we observed some increase in risk for women with younger ages at menarche, although those with very early ages were not at particularly high risk [odds ratio (OR) = 1.5, 95% confidence interval (CI) = 1.1-1.9 for menarche at age 12 and OR = 1.2, 95% CI = 0.9-1.7 for menarche at age ≤ 10]. Women who reported having regular menstrual cycles within 2 years of menarche were at increased breast cancer risk (OR = 1.7, 95% CI = 1.2-2.3), compared to those never having regular cycles. Stratification by current body mass index revealed slightly stronger associations with menstrual characteristics among thinner women (< 22.0 kg/m²) compared to heavier women (> 28.8 kg/m²). Conclusions: These findings suggest that future studies should focus on clarifying how the interrelated effects of body size and menstrual factors, such as age at menarche and cycle regularity, contribute to breast cancer etiology.