Objectives Few studies have investigated the association between the full range of birth weight and the risk of childhood obesity in high, middle and low income countries.The aim of the present study is to assess the association between different levels of birth weight and the risk of obesity among children aged 9-11 years in 12 countries from 6 continents.Methods A multinational, cross-sectional study of 5141 children aged 9-11 years was conducted in 12 countries.Height and weight were obtained using standardized methods.Body mass index (BMI) z-scores were computed using age-and sex-specific reference data from the World Health Organization.Multi-level modeling was used to account for the nested nature of the data.Results The overall prevalence of obesity (BMI z-scores >+2 SD) was 15.4% for boys and 10.0% for girls.There was a positive association between birth weight and BMI z-scores.The multivariable-adjusted (parental education, maternal history of gestational diabetes, child age, sex, infant feeding mode, gestational age, unhealthy and healthy diet pattern scores, moderate-to-vigorous physical activity, sedentary behavior and sleeping duration) odds ratios (ORs) of childhood obesity were significantly higher among children whose birth weights were 3 500-3 999 g (OR 1.45;95% confidence interval (CI): 1.10-1.92), and more than 4 000 g (OR 2.08;95% CI: 1.47-2.93),compared with the reference group (2 500-2 999 g).The positive association between birth weight and the odds of of childhood obesity was seen in girls, while a U-shaped association appeared in boys.Conclusions High levels of birth weight, defined as birth weight ≥3 500 g, were associated with increased odds of obesity among 9-11 year old children in 12 countries.However, sex differences in the association between birth weight and the risk of obesity needs to be considered when planning interventions to reduce childhood obesity.