Preterm birth, body mass index trajectories, and risk of thinness in early childhood: the Japan Environment and Children's study.

This study aimed to examine whether preterm birth is associated with thinness and differences in body mass index (BMI) trajectories at ages 3-6 years. This cohort study used data from the Japan Environment and Children's Study, a nationwide birth cohort. Among 100,121 live births, 93,406 children were included. Preterm birth was defined as < 37 weeks' gestation. Thinness was defined as BMI-for-age < - 2 standard deviations according to World Health Organization criteria. Multivariable logistic regression was used to examine associations between preterm birth and thinness at each age. Longitudinal changes in BMI were analysed using linear mixed-effects models. Preterm birth was associated with higher odds of thinness at most ages: adjusted odds ratios were 1.35 (95% confidence interval [CI], 0.94-1.93) at age 3 years, 1.43 (95% CI, 1.02-1.99) at 4 years, 1.70 (95% CI, 1.29-2.25) at 5 years, and 1.52 (95% CI, 1.20-1.92) at 6 years. Findings were consistent with sensitivity analyses using an alternative definition of thinness. In the longitudinal analysis, mean BMI was consistently lower among preterm-born children, with mean BMI differences of 0.11-0.19 kg/m

conclusionPreterm birth was associated with lower BMI trajectories and higher odds of thinness during early childhood. These findings suggest that incorporating gestational age into growth assessment and monitoring body mass, in addition to linear growth, may help identify children with lower BMI trajectories. WHAT IS KNOWN: • Children born preterm have heterogeneous postnatal growth trajectories, with previous studies reporting both increased risk of obesity and lower body mass during childhood. • Large-scale longitudinal evidence on BMI and thinness during early childhood among children born preterm remains limited. WHAT IS NEW: • In this nationwide birth cohort, preterm birth was associated with higher odds of thinness at ages 4-6 years. • Children born preterm also had consistently lower BMI at ages 3-6 years, with no evidence of convergence with term-born children.