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An instrumental variable approach was used to estimate a causal effect of child BMI z-score on age of menarche. The instrumental variables were pre-pregnancy BMI, high gestational weight gain and smoking during pregnancy. We found strong effects of maternal age of menarche, birthweight, and child BMI z-score (−5.23, 95% CI [−7.35,−3.12]) for both African-Americans and whites. Age of menarche declined with increases in exposure to poverty during early childhood for whites. There was no effect of poverty for African-Americans. We used Oaxaca decomposition techniques to determine how much of the overall race difference in age of menarche was attributable to race differences in observable factors and how much was due to race dependent responses. The African-American/white difference in childhood BMI explained about 18% of the overall difference in age of menarche and birthweight differences explained another 11%.

OBJECTIVES:: Two research questions were addressed in this article: (a) Within each race, at what ages were BMI and height differences evident among the early-, the mid-, and the late-onset groups? And (b) within each timing group, at what ages were BMI and height differences evident between White and African American girls?

METHODS:: The mother/child files of the National Longitudinal Survey of Youth were used for this study. Menarcheal timing groups were identified using the 25th and the 75th percentile of the age distribution for each race. Longitudinal statistical techniques were used to estimate BMI and height as polynomial functions of age and age relative to menarche for African American and White girls.

RESULTS:: Significant differences in BMI by timing group were found. By 3 years of age, significant differences were found between early- and mid-onset African American girls, by 5 years of age between mid- and late-onset African American girls, and by 6 years of age among the three timing groups of White girls. Significant height differences were evident by 5 years of age when comparing early- to mid-onset and mid- to late-onset girls in both race groups. Comparing across race and within timing group, BMI and height differences were evident. African American girls were more likely than White girls to experience accelerated growth and earlier menarche.

DISCUSSION:: This is one of the few longitudinal studies of differences in growth by timing of menarche that includes data on girls younger than 5 years with large samples of both African American and White girls. Understanding when differences are first apparent is critical in establishing the critical period for prevention of these high-risk growth patterns.