Optimal weight gain to reduce obesity risk in preterm infants in a National cohort study.

This study aims to identify the optimal postnatal weight gain through age 6 to prevent obesity in preterm infants. Nested case-control study using a nationwide South Korean cohort (2008–2015) included preterm infants with obesity (cases) and those with BMI between the 5th–85th percentiles (controls) at age 6. Weight-for-age and BMI z-scores were compared using Cohen’s d. Logistic regression identified obesity risk factors, and net benefit analysis determined optimal intervention points by birth weight. Among 41,286 preterm infants, 3349 (8.1%) developed obesity. Birth weight z-score differences between cohorts were small (Cohen’s d = 0.115), but differences in weight-for-age z-scores emerged at 4–6 months (Cohen’s d = 0.474) and grew over time (3 years: Cohen’s d = 1.278). Risk factors included low gestational age, high birth weight, rural residence, lower socioeconomic status, and recent birth. Higher z-scores correlated with greater net benefit, with extremely low birth weight infants showing earlier increases. In conclusion, a rapid increase in z-scores from 4 to 6 months was strongly linked to obesity at 6 years. Optimal predictive values varied by birth weight, emphasizing the need for early monitoring and tailored cut-off values for z-scores based on corrected age to prevent obesity.