Introduction: Three widely referenced growth curves classify infant birth anthropometric measurements as small (SGA), appropriate (AGA), or large (LGA) for gestational age (GA) differently. We assessed how these differences in assignment affect the identification and prediction of neonatal intensive care unit (NICU) mortality risk in US preterm infants. Methods: Birth data of infants admitted to NICUs from the Pediatrix Clinical Data Warehouse (2013–2018) were analyzed. Birth weight, length, and head circumference of 46,724 singleton infants (24–32 weeks GA) were classified as SGA, AGA, or LGA using the Olsen, Fenton, and INTERGROWTH-21st curves. NICU mortality risk based on birth size classification was analyzed using unadjusted and adjusted logistic regression stratified by GA. Results: Odds of mortality were increased with SGA classification at all GAs, size measurements, and curve sets, compared with AGA infants. LGA classification for weight was associated with lower mortality risk at 24 weeks GA and higher risk at 30 weeks GA. Odds of mortality did not differ significantly across curve sets. Classification of size at birth alone had relatively low predictive ability to identify mortality risk, with unadjusted AUCs near 0.5 for all analyses. Conclusion: There were no significant differences across curve sets in predicting mortality. Classification of size at birth is a relatively imprecise method to identify infants at risk for NICU mortality.

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