Relative weight and race influence average age at menarche: results from two nationally representative surveys of US girls studied 25 years apart.

OBJECTIVES 1) To establish with nationally representative US data whether menarche occurred earlier in the 1990s than it had 25 years before. 2) To assess whether the occurrence of menarche in relation to weight status and race had changed over this time period. METHODS Relative weight, race, and menarcheal status of girls in the National Health Examination Survey cycles II and III (1963-1970) were compared with results from the Third National Health and Nutrition Examination Survey (1988-1994). Probit analysis was used to determine the average age at menarche during the 2 survey periods. Logistic regression was used to assess the association of relative weight to likelihood of having reached menarche. RESULTS The average age at menarche dropped from 12.75 to 12.54 years, and the percentage of girls between 10 and 15 years old who were above the 85th percentile for body mass index increased from 16% to 27% over the 25 years between the 2 surveys. Higher relative weight was strongly associated with increased likelihood of having reached menarche after controlling for age and race. Black girls had a lower average age at menarche than did white girls, which was independent of the effect of relative weight. CONCLUSIONS These analyses from 2 nationally representative samples of US girls suggest a drop of about 2(1/2) months in the average age of menarche during the time period between 1963-1970 and 1988-1994. This was paralleled by a concurrent shift in the population distribution of body mass index z-score toward higher relative weights.