Studies of the association between breast size, as a proxy for mammary gland mass, and breast cancer risk have given equivocal results. Most have been case-control studies with limited statistical power. We conducted a prospective analysis of the relation between breast size as measured by self-reported bra cup size and breast cancer risk among premenopausal women enrolled in the Nurses' Health Study II. Bra cup size at age 20 was assessed among 89,268 premenopausal women aged 29-47 in 1993. Subsequent incident cases of invasive breast cancer were assessed until 2001. Hazard ratios (HRs) and 95% confidence intervals (CIs) were calculated with a Cox proportional hazards model adjusting for potential confounders and risk factors for breast cancer. During 622,732 person-years of follow-up, 803 premenopausal women were newly diagnosed with invasive breast cancer. For women with a BMI below 25 kg/m2, those with a bra cup size of "D or larger" had a significantly higher incidence of breast cancer than women who reported "A or smaller" (covariate adjusted HR=1.80; 95% CI 1.13-2.88; ptrend=0.01). There was no significant association among women with a BMI of 25 kg/m2 or higher. Stratifying by BMI at age 18 at a cutoff point of 21 kg/m2 gave similar results. Larger bra cup size at a young age is associated with a higher incidence of premenopausal breast cancer, though this association is limited to leaner women.
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