Prenatal diethylstilbestrol exposure and mammographic density.

In a prospective cohort study of the health effects associated with prenatal Diethylstilbestrol (DES) exposure, DES was associated with an increased breast cancer risk after 40 years of age. It is unknown whether it is associated with greater mammographic density, which strongly predicts breast cancer risk. A cohort of DES-exposed and unexposed women was assembled at the Mayo Clinic in 1975, and followed through 2012 as part of the National Cancer Institute's DES follow-up study. Mammographic density from 3,637 mammograms for 332 (222 DES-exposed, 110 unexposed) women in this cohort screened at the Mayo Clinic, Rochester between 1996 and 2015 was determined clinically using the Breast Imaging Reporting and Data System (BI-RADS). Any effect of prenatal DES exposure on mammographic density was estimated using repeated measures logistic regression. There was no association between prenatal DES exposure and high mammographic density for either premenopausal [Odds ratios (OR) = 0.92 (95% Confidence Interval (CI): 0.50, 1.7] or postmenopausal women (OR = 0.90; 95% CI: 0.54, 1.5). Among premenopausal women, associations differed by body mass index (BMI), with ORs of 1.47 (0.70, 3.1) for women with BMI above the median and 0.53 (0.23, 1.3) for those with BMI below the median (pinteraction  = 0.05). Overall, however, prenatal DES exposure was not associated with high mammographic density in this sample of DES Study participants. Consequently, this study does not provide evidence that high mammographic density is involved with the influence of DES on breast cancer risk.