Expected gain in body mass and onset of the menarche.

Fertility is dependent on sexual maturity, which is associated with a number of factors, including body mass. In this study the Body Mass Index (BMI) was used to estimate body fat in a large sample of 13-year-old New Zealand girls. Nearly half of the subjects (46%) had experienced the menarche. Expected gain in body mass was calculated from growth rates at earlier ages and used to examine whether deviation from the expected gain was associated with the onset of the menarche. In general, the results show an association between the menarche and gain in body mass. However, many girls who failed to achieve their expected gain had experienced the menarche (18%), indicating that the relationship between body weight and the menarche may not be causal or is mediated by other factors.