Secular trends in age at menarche in 20th century Mexico: Differences by ethnicity, area of residency, and socioeconomic status.

Objectives: This study aims to assess the secular trend in age at menarche (AAM) in Mexico over the 20th century, and compare the patterns according to area of residence (rural/urban), socioeconomic status (SES), and ethnicity (indigenous/nonindigenous).

Methods: Data on AAM from 24 380 women aged ≥20 years born between 1906 and 1986 were obtained from the Mexican National Health and Nutrition Survey 2006. Birth cohorts were compared to test for a secular trend and differences in mean AAM by area of residence, SES, and ethnicity were evaluated using the Welch test for heterogeneous variances followed by Tamhane T for post hoc comparisons.

Results: Mean AAM declined from 13.3 years among Mexican women born before the 1940s to 12.56 years among those born in the 1980s. Across birth cohorts, urban women had significantly earlier AAM than their rural counterparts. Nonindigenous urban women reached menarche the earliest and rural indigenous women the latest of all groups. Nonindigenous urban residents experienced a comparatively earlier decline, while that for the indigenous rural women occurred last. High SES women reached menarche the earliest and low SES women the latest. The historical decline in AAM for high and medium SES groups occurred relatively early, whereas that for the low SES occurred last.

Conclusions: Mean AAM was associated with area of residence, ethnicity, and SES. Our findings indirectly suggest that advances in living conditions experienced in Mexico during the 20th century appear to have been insufficient to overcome the social and biological inequalities accumulated over centuries in some groups.