Temporal trends in age at menarche among females born between 1931 and 2005 in a us-based digital cohort

Previous cross-sectional studies have noted declining age at menarche over the past 5 decades, but data is still limited for exploring the differences in this trend by other factors, such as race/ethnicity, region, and health conditions. We explored these trends in the Apple Women’s Health Study (AWHS). We conducted a retrospective analysis within a subgroup of 71,700 participants in the AWHS who enrolled between November 14, 2019 and March 19, 2023, having met the study eligibility criteria upon enrollment, and provided an answer to the age at menarche question (range: 7-16 years) in the Reproductive History Survey. Birth year was grouped into the following categories: 1930-1969, 1970-1979, 1980-1989, 1990-1999, or 2000-2005. Covariates preceding menarche included: race/ethnicity, family history of polycystic ovary syndrome (PCOS), and whether born preterm. Characteristics at enrollment were: geographical location, educational attainment, and history of PCOS and infertility. Temporal trends of age at menarche by birth year by 5-year intervals were graphically visualized. We further evaluated the differences in temporal trends of (1) the mean age at menarche, and (2) the proportion of participants with age at menarche < 11 years, differed by the above mentioned covariates (an interaction term of each covariate*birth year was used to determine statistical significance for the temporal trend). Participants’ self-reported birth year at enrollment ranged from 1931 to 2005. The visualizations showed a decline in mean age at menarche over the birth year categories (from 12.5 years in 1930-1969 to 11.9 years in 2000-2005). The proportion with age at menarche < 11 years also increased over the birth year categories (from 9% in 1930-1969 to 16% in 2000-2005). On average, age at menarche was earlier among non-Hispanic Black or Hispanic participants, compared to non-Hispanic white participants. The difference in mean age at menarche comparing each race/ethnicity group to the non-Hispanic white group increased over time (p-trend = 0.03 for non-Hispanic Black, = 0.04 for Hispanic, = 0.02 for Asian, and <0.01 for other/more than one race/ethnicity). Those without a college degree at enrollment had a higher proportion of earlier age at menarche compared to those with a college degree. The difference in mean age at menarche comparing those without vs. with a college degree also increased over time (from -0.07 years in 1930-1969 to -0.33 years in 2000-2005, p-trend < 0.0001). There was no statistically significant temporal trend in the difference in age at menarche when stratified by PCOS or family history of PCOS, whether born preterm, geographical location, or infertility. We found a decline in age at menarche over the birth years from 1931 to 2005, and the magnitude of this decline was larger among non-white individuals or those with lower education attainment. Future studies should further examine this trend and potential health implications.