We compared the median age at menarche in two developing countries to determine whether early age at menarche varies across body mass index (BMI), socioeconomic level, and urban/rural residence in Mexican and Egyptian adolescents. Questionnaire data and weight and height measurements were collected from 4,143 Mexican and 493 Egyptian adolescent girls (age, 10-19 years). Statistical analysis included Pearson chi-squared, Student t-test, and multiple logistic regression models. Median age at menarche was 12 years (range, 8-19 years) for Mexican girls and 13 years (range, 9-16) for Egyptian girls. In both countries, the odds of undergoing menarche at an earlier age were higher among adolescents with the following characteristics: overweight or obesity, as compared to those with a normal BMI (Mexican adolescents, OR = 1.45, 95% CI 1.25-1.69; Egyptian adolescents, OR = 2.20, 95% CI 1.21-4.03); living in urban versus rural areas (OR = 1.38, 95% CI 1.19-1.61 and OR = 1.38, 95% CI 0.74-2.59, respectively); high socioeconomic level versus the lowest level (OR = 1.46, 95% CI 1.22-1.74 and OR = 1.41, 95% CI 0.67-2.98, respectively). The associations between early age at menarche and urban residence or high socioeconomic level were statistically significant only in Mexican adolescents. Menarche at an earlier age was most strongly associated with overweight and obesity in both Mexican and Egyptian adolescents, while urban residence and high socioeconomic level were significantly associated only in Mexican adolescents.
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