Secular trends and ethnic disparities in age at menarche among ethnic minority girls in China: a 35-year nationwide repeated cross-sectional study from 1985 to 2019

Background: Age at menarche (AAM) is a critical marker of female development and long-term health. While its global decline has been documented, its developmental trajectories and heterogeneity among China's large ethnic minority populations remain poorly characterised. We sought to explore the temporal trends and differences in AAM between 26 major ethnic minority groups and their Han peers in China, and to examine the role of economic development in shaping these patterns. Methods: We retrieved data on 190,064 girls from seven waves (1985-2019) of the Chinese National Survey on Students' Constitution and Health, covering 26 minority groups and local Han peers across 13 provincial-level regions. We estimated the median AAM and its 95% confidence interval (CI) via probit regression based on status quo menarcheal data. We assessed intergroup differences using Z-tests, identified distinct trajectories of differences in AAM between 25 minority groups and their local Han peers through K-means clustering; and examined associations between AAM and log-transformed prefecture-level per capita GDP using quadratic regression models. Results: The median AAM among minority girls in 2019 was 12.54 years, later than that of Han girls (12.05 years), with an overall difference of 0.49 years and substantial interethnic heterogeneity. The AAM declined across all minority groups from 1985 to 2019, albeit at different rates. Three trajectories of minority-Han differences were identified: a 'rapid catch-up' pattern (n/N = 13/25 groups), an 'emerging divergence' pattern (n/N = 8/25 groups), and a 'fluctuating/stable' pattern (n/N = 4/25 groups). These clusters exhibited a clear gradient in regional economic development. Economic growth showed a nonlinear association with AAM, with stronger declines at lower development levels and a plateau at higher levels. Conclusions: Although regional economic development was associated with AAM among ethnic minorities in China, convergence with Han peers remains uneven. The nonlinear association with economic growth suggests diminishing returns of development, underscoring the need for targeted public health strategies addressing both persistent deprivation and modernisation-related risks.