Association Between Baseline Mammographic Breast Density and the Incidence of Natural Menopause in Korean Women: A Nationwide Population–Based Study Using National Health Screening Data

Background: High mammographic breast density (MBD) reflects cumulative exposure to sex hormones. While it is well established that breast density declines during the menopausal transition, the association between baseline MBD and the subsequent timing of natural menopause remains underexplored. We investigated the relationship between baseline MBD and incident natural menopause in Korean women. Methods: We conducted a retrospective cohort study using the Korean National Health Insurance Service database. A total of 428,998 premenopausal women aged 40– 55 years who underwent breast cancer screening in 2009 were included. Baseline MBD was classified using the BI–RADS system. Incident natural menopause was defined by consistent self–reports across at least two subsequent screenings (2011– 2013). Multivariate logistic regression models were used to estimate odds ratios (ORs) for incident menopause, adjusting for confounders including age, BMI, and reproductive history. Results: A total of 73,599 women (17.16%) experienced natural menopause. Higher MBD was associated with progressively lower odds of incident menopause. Women with extremely dense breasts exhibited the lowest likelihood of reaching menopause (adjusted OR 0.8, 95% CI 0.77– 0.83), indicating a delayed menopausal transition. Stratified analyses revealed that the inverse association was more pronounced in women age < 50 years and those with late menarche (≥ 13 years). Conclusion: High premenopausal MBD was independently and inversely associated with the incidence of natural menopause, suggesting a potential link between dense fibroglandular tissue and variations in reproductive aging. Clinically, baseline MBD could potentially serve as a readily available indicator to evaluate reproductive aging patterns, aiding in more personalized health counseling regarding reproductive health and surveillance strategies.