Contemporary age at menarche among Thai girls: a nationwide cross-sectional study and its public health implications

Purpose: Age at menarche has declined globally. However, contemporary population-level data from Southeast Asia remain limited. This study aimed to characterize menarcheal timing among Thai schoolgirls and identify associated factors using survival-based methods.Methods: A nationwide cross-sectional study was conducted among 8,161 Thai schoolgirls in grades 3–9. Menarcheal status and age at menarche were assessed using questionnaires and verified by caregivers. Kaplan-Meier survival analysis was used to estimate the cumulative probability of menarche by age, accounting for right-censoring. Determinants of menarcheal timing were examined using Cox proportional hazards regression models.Results: At assessment, 52.6% of participants had attained menarche. Kaplan-Meier analysis demonstrated that the cumulative probability of menarche rose from 5.56% at 10 years to 85.21% at 13 years, with a median age of 12.0 years (interquartile range, 11.2–12.7 years). Based on survival-derived percentiles, early menarche (approximately <5th percentile) was defined as occurring before 10 years of age. Significant regional differences in menarcheal timing were observed (log-rank test, <i>P</i><0.001). In multivariable Cox regression analysis, overweight (hazard ratio [HR], 1.67), obesity (HR, 1.64), and high socioeconomic status (HR, 1.28) were independently associated with earlier menarche, whereas underweight status was associated with delayed menarche (HR, 0.45; all <i>P</i><0.001). After adjustment, only the Southern region remained significantly associated with later menarche compared with Bangkok.Conclusion: Nutritional status and socioeconomic factors are key factors associated with menarcheal timing in Thailand. Survival-based, percentile-defined thresholds provide a useful framework for identifying atypical pubertal timing and may inform population-level public health strategies.