RESUMO
Background: The age at menarche may influence decisively health and disease in women. It also indicates the beginning of the reproductive period and, as a consequence, the possibility of biological continuity for the human species. Genetic and environmental determinants define the age of menarche and can explain differences found among different populations. Aim: To determine the age at menarche among adolescents with different levels of indigenous descent (parental indigenous surnames), considering the effect of socioeconomic and demographic factors. Material and Methods: An observational study of historic cohorts of8.624 girls from the Arauca-nía Region (central-southern Chile) was carried out. Data were collected by health professionals using a previously validated questionnaire. Occurrence of menarche was estimated through survival analysis and compared between groups (according to indigenous parental surnames) adjusted for parents' income and educational level and provenance (rural/urban). Results: Estimated median age of menarche was 151 months (95% Cl: 150-151). In female with four indigenous surnames, menarche occurred two months later than girls without indigenous surnames and with two indigenous surnames (p < 0,001). In girls whose parents had lowest level of schooling, the difference increased to eight months later (p < 0,005). Conclusions: Age at menarche in the group with higher indigenous descent is later even if socio-economic conditions remain stable. Genetic factors might play an important role, however conditions of vulnerability can influence and further delay the onset of reproductive competency.