IMPORTANCE: Patterns of body size and body composition associated with genetic obesity susceptibility inform the mechanisms that increase obesity risk. OBJECTIVE: To test associations between genetic obesity susceptibility, represented by a combined obesity risk-allele score, and body size or body composition at birth to age 5 years. DESIGN, SETTING, AND PARTICIPANTS: A total of 3031 children from 4 birth cohort studies in England, France, and Spain were included in a meta-analysis. EXPOSURES: A combined obesity risk-allele score was calculated from genotypes at 16 variants identified by genome-wide association studies of adult body mass index (BMI). MAIN OUTCOMES AND MEASURES: Outcomes were age- and sex-adjusted SD scores (SDS) for weight, length/height, BMI, fat mass, lean mass, and percentage of body fat at birth as well as at ages 1, 2 to 3, and 4 to 5 years. RESULTS: The obesity risk-allele score was not associated with infant size at birth; at age 1 year it was positively associated with weight (β [SE], 0.020 [0.008] SDS per allele; P = .009) and length (β [SE], 0.020 [0.008] SDS per allele; P = .01), but not with BMI (β [SE], 0.013 [0.008] SDS per allele; P = .11). At age 2 to 3 years these associations were stronger (weight: β [SE], 0.033 [0.008] SDS per allele; P < .001; height: β [SE], 0.025 [0.008] SDS per allele; P < .001) and were also seen for BMI (β [SE], 0.024 [0.008] SDS per allele; P = .003). The obesity risk-allele score was positively associated with both postnatal fat mass (1 year: β [SE], 0.032 [0.017] SDS per allele; P = .05; 2-3 years: β [SE], 0.049 [0.018] SDS per allele; P = .006; and 4-5 years: β [SE], 0.028 [0.011] SDS per allele; P = .009) and postnatal lean mass (1 year: β [SE], 0.038 [0.014] SDS per allele; P = .008; 2-3 years: β [SE], 0.064 [0.017] SDS per allele; P < .001; and 4-5 years: β [SE], 0.047 [0.011] SDS per allele; P < .001), but not with the percentage of body fat (P > .15 at all ages). CONCLUSIONS AND RELEVANCE: Genetic obesity susceptibility appears to promote a normally partitioned increase in early postnatal, but not prenatal, growth. These findings suggest that symmetrical rapid growth may identify infants with high life-long susceptibility for obesity.