Body mass index (BMI) trajectories in infancy differ by population ancestry and may presage disparities in early childhood obesity.

Context: No consensus definition exists for excess adiposity during infancy. After age 2 years, high body mass index (BMI) is related to adverse cardiometabolic outcomes. Before age 2 years, the utility of BMI as a metric of excess adiposity is unknown.

Objectives: The objective of the study was to characterize infant BMI trajectories in a diverse, longitudinal cohort and investigate the relationship between the infancy BMI trajectory and childhood obesity.

Subjects: Healthy, nonpreterm infants (n = 2114) in the Genetic Causes for Complex Pediatric Disorders study (The Children's Hospital of Philadelphia) with six or more BMI measurements in the first 13.5 months participated in the study.

Design: For each infant, the BMI trajectory was modeled using polynomial regression. Independent effects of clinical factors on magnitude and timing of peak BMI were assessed. The relationship between infancy BMI and early childhood BMI (age 4 y) was examined (n = 1075).

Results: The cohort was 53% male and 61% African-American. Peak BMI was 18.6 ± 1.7 kg/m(2) and occurred at 8.6 ± 1.4 months. In multivariate analysis, boys had a higher (0.50 kg/m(2), P < .001) peak BMI than girls. The peak was higher (0.53 kg/m(2), P ≤ .001) and occurred earlier (by 12 d, P < .001) in African-American vs white children. The odds of obesity at age 4 years increased among children with higher (odds ratio 2.02; P < .001) and later (odds ratio 1.26; P = .02) infancy peak BMI.

Conclusions: We demonstrate sex- and ancestry-specific differences in infancy BMI and an association of infancy peak BMI with childhood BMI. These findings support the potential utility of infancy BMI to identify children younger than age 2 years with increased risk for later obesity.