title: Interpreting the continued decline in the average age at menarche: results from two nationally representative surveys of U.S. girls studied 10 years apart.
abstract: OBJECTIVE
  To determine whether average age at menarche declined in the United States during the past decade, and whether associations between menarcheal timing, weight status, and race/ethnicity changed.
  
  
  METHODS
  Relative weight, race/ethnicity, and menarcheal status of girls (n = 1577) in the National Health and Nutrition Examination Survey (NHANES) III (1988-1994) were compared with those of girls (n = 1720) in NHANES 1999-2002. Probit analysis estimated average age at menarche overall and also by race/ethnicity. Logistic regression assessed associations of relative weight and race/ethnicity with menarcheal status.
  
  
  RESULTS
  In the United States, average age at menarche declined from 12.53 years (95% confidence interval [CI] = 12.43 to 12.63 years) in 1988-1994 to 12.34 years (95% CI = 12.24 to 12.45 years) in 1999-2002. By race/ethnicity, average age at menarche estimates were as follows: non-Hispanic whites, 12.57 years (95% CI = 12.45 to 12.69 years) and 12.52 years (95% CI = 12.38 to 12.67 years); non-Hispanic blacks, 12.09 years (95% CI = 11.82 to 12.36 years) and 12.06 years (95% CI = 11.81 to 12.32 years); and Mexican Americans, 12.24 years (95% CI = 11.88 to 12.59 years) and 12.09 years (95% CI = 11.81 to 12.37 years). Higher relative weight was consistently associated with increased likelihood of having reached menarche.
  
  
  CONCLUSIONS
  Average age at menarche in the United States declined by 2.3 months between 1988-1994 and 1999-2002; by race/ethnicity, declines were considerably smaller. Changes in the population distribution of race/ethnicity and relative weight should be considered when interpreting trends in age at menarche.
authors.affiliation: Gerald J. and Dorothy R. Friedman School of Nutrition Science and Technology and the Department of Public Health and Family Medicine, Tufts University, Boston, Massachusetts 02111, USA.
authors.forename: Sarah E
authors.initials: SE
authors.lastname: Anderson
authors.forename: Aviva
authors.initials: A
authors.lastname: Must
mesh_terms: D000293:Adolescent; D001741:Black or African American / Q000706:statistics & numerical data; D017677:Age Distribution; D015992:Body Mass Index; D002648:Child; D005260:Female; D006801:Humans; D016015:Logistic Models; D008572:Menarche / Q000208:ethnology / Q000502:physiology*; D017058:Mexican Americans / Q000706:statistics & numerical data; D014481:United States; D044465:White People / Q000706:statistics & numerical data