Agreement between Fenton and intergrowth curves in assessing birth weight of preterm infants: Bland-Altman analysis by degree of prematurity and birth weight for gestational age

OBJECTIVE: To assess the agreement between the Fenton and INTERGROWTH charts in classifying birth weight for gestational age in preterm infants, considering prematurity degree and small/adequate/large-for-gestational-age (SGA/AGA/LGA) status individually and simultaneously. METHODS: Retrospective cohort study including 2529 preterm infants admitted between 2018 and 2021. Gestational age was estimated from obstetric data or by the Capurro/Ballard methods. Birth weight-for-gestational-age Z-scores were calculated according to both charts. Agreement was analyzed overall and by subgroups defined by prematurity degree and SGA/AGA/LGA status using the Bland-Altman method, complemented by boxplots. RESULTS: Overall agreement was reasonable (mean bias: -0.08; limits of agreement: -0.67 to 0.50), with slight overestimation of Z-scores by INTERGROWTH. Relevant variability was observed across subgroups. Disagreement extended across all prematurity strata with distinct patterns: greater dispersion in extremely and very preterm infants and more consistent but systematically biased differences in moderate-to-late preterm infants (bias: -0.12). The largest discrepancies occurred among SGA, especially in extremely preterm infants, among whom 50% had Z-scores beyond the limits of agreement. In this subgroup, Fenton tended to assign higher Z-scores, especially between 26 and 32 weeks of gestation, attenuating birth weight deficit severity. Agreement was considerable among AGA and LGA infants, especially in more mature preterm groups. CONCLUSION: Although overall agreement between the charts was acceptable, clinically relevant differences were observed across all prematurity strata, particularly among more immature SGA infants. The charts are not interchangeable at the individual level, and their use may influence nutritional diagnosis and clinical management, reinforcing the importance of consistent and context-appropriate selection of growth reference.