Determinants of BMI category changes during middle childhood and adolescence in girls.

The aim of the study was to analyse the changes in BMI category between 7 and 16-18 years of age on the basis of continuous studies and evaluate which factors are related to the increase and decrease in BMI category. Data were collected from 1008 secondary school students aged 16-18 y. Body height and weight were measured and BMI was calculated. Information on height and body mass in earlier periods of life was retrieved from medical records. Age at menarche and socio-economic status was established on the basis of a questionnaire. Most girls who were underweight at 7 y. as well as those who were overweight or obese at 7 y. at the age of 16-18 y. reach correct body weight. The probability of the BMI category changing to higher one at 16-18 y. was related to short stature at 7, 9 and 14 y., low socio-economic status, living in the village and primary level of mother education. A drop in the BMI category was linked to the incidence of overweight and obesity at the age of 9 and 14 y. and tall stature at the age of 7 and 9 years. High socio-economic status was connected with a change in the category from "correct weight" at the age of 7 y. to "underweight" at 16-18 y., whereas the socio-economic status did not have a significant effect on the change of the category "overweight & obesity" at 7 y. to "correct weight" at 16-18 y.