Comparison of the 2025 Third-Generation and 2013 Fenton Growth Charts in Preterm Infants Born before 35 Weeks: A Retrospective Cohort Study

Objective: This study aimed to compare the classification of birth size using the Fenton 2025 third-generation growth charts versus the widely used Fenton 2013 charts in preterm infants born before 35 weeks' gestation. Study Design: This retrospective, observational cohort study included preterm infants born at <35 weeks' gestation and admitted to a tertiary-level neonatal intensive care unit (NICU) between January 2019 and August 2025. Birth weight, head circumference (HC), and length were classified using sex-specific Fenton 2013 and Fenton 2025 growth chart calculators. Infants were categorized as small for gestational age (SGA, <10th percentile), appropriate for gestational age (AGA, 10th-90th percentile), or large for gestational age (LGA, >90th percentile). Agreement between chart versions was assessed using weighted Cohen's kappa, and marginal distributions were compared using the Stuart-Maxwell test. Results: = 0.001), reflecting a downward shift in birth weight z-scores and reclassification of 13.2% of infants from AGA to SGA. Agreement was stronger for HC (93.5%, κ = 0.80) and length (96.3%, κ = 0.90), with minimal reclassification. Classification differences varied by gestational age, with greater shifts observed at later preterm gestations. Conclusion: In preterm infants born before 35 weeks' gestation, the Fenton 2025 growth charts demonstrate high overall agreement with the Fenton 2013 charts but identify a substantially higher proportion of infants as SGA based on birth weight. These differences may have important clinical and research implications and warrant careful consideration. Key Points: · Fenton 2025 identifies more SGA preterm infants.. · Birth weight shows the greatest classification change.. · Reclassification of birth weight predominates around the 10th percentile..