Generational trends in reproductive factors among women in the US: implications for breast cancer incidence

Abstract Background Reproductive factors are key breast cancer risk factors, yet contemporary generational patterns remain unclear. We aimed to evaluate trends in reproductive factors among US women born between 1910 and 2000. Methods We conducted a serial, cross-sectional analysis of reproductive factors using data from the US National Health and Nutrition Examination Survey (NHANES) collected between 1999 and 2020. Female participants were grouped by birth cohorts (1910–1930, then by 10-year to 2000). Self-reported data were used to derive age at menarche, prevalence of age at menarche < 12 years, age at natural menopausal, prevalence of first live birth after age 30, and lifetime number of live births. Results Data on 28,481 US women were analyzed. From birth cohort 1910–1930 to 1990–2000, the mean age at menarche declined from 13.0 (95%CI 12.9 to 13.1) to 12.4 (95%CI 12.3 to 12.5) years (difference = − 0.6, 95%CI − 0.7 to − 0.5, p for trend < 0.001). This decline was observed in both US (12.9 95%CI 12.9 to 13.0 to 12.4 95%CI 12.3 to 12.5) and non-US born (13.3 95%CI 13.1 to 13.6 to 12.3 95%CI 12.1 to 12.5) women. From birth cohort 1910–1930 to 1990–2000, the prevalence of age at menarche < 12 years increased from 14.2% (95%CI 12.4 to 16.1) to 26.5% (95%CI 24.3 to 28.9%). The prevalence of first live birth after age 30 increased from 4.7% (95%CI 3.1 to 6.9%) to 10.6% (95%CI 7.3 to 15.0%) from birth cohort 1910–1930 to 1980–1990. Among women who attained menopause (up to birth cohort 1950–1960), no significant changes were observed for age at natural menopausal. However, the average lifetime number of live births declined from 3.5 to 2.4 with a significant decline in the proportion of grand multiparous women. Conclusions Over the past century, women in the US are attaining menarche earlier, more likely to have their first birth after age 30 and having fewer births. Parsing the extent to which changes in reproductive factors are contributing to the rising incidence of estrogen receptor positive breast cancer, especially in premenopausal women, is necessary to devise long-lasting and sustainable preventive strategies.