Association of number of children and birth order with body mass index in children and adolescents: The CASPIAN-V study

Abstract Background This study aimed to evaluate the association of the number of family children and birth order with later body mass index (BMI) in a pediatric population. Methods This cross-sectional nationwide study was conducted in the framework of the fifth survey of a national school-based surveillance program, entitled Childhood and Adolescence Surveillance and PreventIon of Adult Noncommunicable disease (CASPIAN-V) Study. Participants were 14,400 students, aged 7–18 years, who were selected by random cluster sampling from 30 provinces in Iran. Multinomial regression models were used to evaluate the odds ratio (OR) and 95% confidence intervals (95% CI) of the number of family children or the birth order for later excess weight. Results The first-born children had significantly higher OR for obesity, and the third-born children had higher OR of underweight compared to the first-born ones (OR 1.22, 95% CI 1.05–1.42). Moreover, children within the three-, four-, and five-child families had higher OR of underweight than those with one-child families (OR 0.88, 95% CI 0.77- 1). Conclusion The birth order and number of children were associated with underweight and overweight or obesity in children and adolescents. Future studies are necessary to assess the underlying determinants.