Age at Menarche Among Mizo Women in Aizawl, Mizoram: A Secular Trend Across Five Birth-Decade Cohorts (1960-2007)

BACKGROUND: Age at menarche (AAM) is a key indicator of female pubertal development. A secular decline in AAM has been observed globally, but data from indigenous populations of Northeast India remain limited. This study documents the secular trend in AAM among Mizo ethnic women over five decades and examines its association with pre-menarcheal residential setting. METHODS: A cross-sectional retrospective study was conducted among 2,178 Mizo women currently residing in Aizawl and born between 1960 and 2007, classified into five birth-decade cohorts. AAM was obtained through structured bilingual questionnaires. Pre-menarcheal residential setting (rural or urban) was based on self-reported childhood residence. Linear regression was the primary analysis; one-way ANOVA and nonparametric tests served as confirmatory analyses. Multivariable regression and bootstrap resampling were used to test sensitivity to confounding and unequal cohort sizes. RESULTS: The overall mean AAM was 13.50±1.49 years. A significant declining trend was observed across cohorts (F(4,2173)=54.587, p<0.001, η²=0.091): mean AAM decreased from 14.67±1.19 years (1960-1969) to 13.09±1.45 years (2000-2007), a decline of approximately 0.40 years per decade. When restricted to the three most recent cohorts (1980-2007), where recall intervals were shorter, the decline remained significant (0.30 years per decade). Women with rural pre-menarcheal residence had later AAM than those with urban residence (13.76 vs. 13.34 years; Cohen's d=0.280), though the effect was small. In multivariable regression, the year-of-birth effect remained significant after covariate adjustment (B=-0.034, p<0.001). The prevalence of early menarche (≤12 years) increased from 4.0% to 35.2% across cohorts (p<0.001). CONCLUSIONS: A significant secular decline in AAM has occurred among Mizo women in Aizawl over five decades, consistent across analytical approaches and residential strata. The concurrent rise in early menarche prevalence carries public health relevance given Mizoram's high noncommunicable disease burden. Although generalizability to rural Mizoram is limited, these findings add to the limited evidence on menarcheal timing among indigenous ethnic groups in Northeast India and underscore the need for prospective studies linking pubertal timing to later-life disease outcomes.