Growth charts are essential for monitoring the postnatal growth of preterm infants. The preterm postnatal follow-up study (PPFS) of the Intergrowth-21st Project provides new growth standards based on a longitudinal study. This study was conducted to investigate the prevalence of extrautrine growth restriction (EUGR) and the associated factors of EUGR in preterm infants, using the PPFS charts and the Fenton charts. Data of 1,356 infants with gestational age (GA) less than 28 weeks from the Korean Neonatal Network were analysed. The prevalence of small for gestational age (SGA) of weight and length was higher with the Intergrowth charts than with the Fenton charts. EUGR in weight and length was more prevalent when using the Fenton charts. Multivariate analysis showed that low GA, high birthweight z score, male, treated patent ductus arteriosus (PDA), necrotizing enterocolitis, intraventricular haemorrhage and duration of parenteral nutrition (PN) were associated with EUGR in weight by the Intergrowth charts. High birthweight z score, treated PDA and PN duration were associated with EUGR defined by the Fenton charts.Conclusion: Compared to the Fenton charts, SGA was more defined and EUGR was less prevalent in extremely low gestational infants, while EUGR defined by the Intergrowth charts categorized infants with adverse clinical courses more elaborately. What is Known: • Preterm infants are at risk of postnatal growth restriction (PGR), although optimal postnatal growth is important for the long-term outcomes. • Growth charts are essential tools to monitor the postnatal growth of preterm infants. What is New: • PGR of weight and length were less defined with the Intergrowth charts than the Fenton charts. • PGR defined by the Intergrowth preterm postnatal follow-up study (PPFS) chart categorized preterm infants with morbidities more elaborately than the Fenton charts.