A Description of Temporal Trends and Age-Period-Cohort Effects in the Association Between Age at Menarche and Adult Obesity Among Women in the United States

Background: In recent decades, there have been concurrent secular trends in decreasing age at menarche and increasing prevalence of obesity. Objectives: To describe temporal trends and age-period-cohort effects in the association between age at menarche and adult BMI using population-representative data from the U.S. National Health and Nutrition Examination Survey (NHANES, 1999–2018). Methods: Self-identified female participants ≥ 18 years completed a Reproductive Health Questionnaire and physical examination. Age at menarche (continuous, years) was self-reported, and body mass index (BMI) (continuous, kg/m²) was calculated using measured height and weight. Age at menarche and adult BMI were characterised using descriptive statistics by age (at the time of the survey), period (NHANES data collection cycle), and birth cohort (10-year intervals). Linear regression was used to assess the association between age at menarche and adult BMI, overall and stratified according to age, period and cohort. Results: Of 23,923 NHANES participants ≥ 18 years between 1999 and 2018, mean adult BMI was greatest among individuals who self-reported age at menarche ≤ 11 years (30.9 kg/m² ± 8.1). Mean adult BMI was highest among individuals with earlier ages at menarche across age, period, and cohort. In linear regression analyses, there were greater increases in adult BMI per 1-year decrease in age at menarche among individuals who reported earlier ages at menarche (β 1.80 kg/m², 95% CI 1.44, 2.16), with larger associations among younger age groups and recent birth cohorts. Conclusions: The present work characterises secular trends in decreasing age at menarche and increasing obesity prevalence to demonstrate age and cohort effects in the association between age at menarche and adult BMI. Identification of these age and cohort effects helps detect populations at risk and informs obesity prevention strategies, highlighting age at menarche as a valuable early-life indicator of obesity risk across the life course.