Birth order and BMI in teenage girls. 
The goal of this study was to investigate the relation of birth order to relative weight and prevalence of obesity in a group of 13-15 years old girls.  In 1997, 1458 girls were examined.  The height and weight measured by trained staff were recorded.  Family size and birth order were obtained by a questionnaire.  For the purpose of the present study, 776 and 250 girls coming from two- and three-child full families, respectively, were selected from the total sample on the basis of complete information.  The Body Mass Index (kg/m2) was adjusted to reference US population (NCHS) by means of the LMS parameters.  Prevalence of overweight and obesity was defined according to recommendation of the International Obesity Task Force.  The effect of birth order on BMI was tested by one-way analysis of variance.  Prevalence of obesity was tested by the means of Pearson chi-square.  First and second born girls from two-sibling families did not show significant differences in average standardized BMI.  Relative weight significantly differs among girls coming from three sibling families, decreasing along with the birth order.  The first-born girls were 1.5 times at higher risk of obesity in comparison to later-born girls.  Differences in the proportion of overweight girls among birth order groups showed a high significance within three sibling families.