Background: Weight z scores at 36 weeks of postmenstrual age (PMA) define postnatal growth failure (PGF) and malnutrition. This study aimed to determine weight z scores at 36 weeks PMA that are associated with adverse cognitive outcomes at 2 years of age. Methods: In this retrospective cohort study, 350 infants 24–26 weeks of gestation born between 2006 and 2014 and followed at 2 years were included. Weight z scores at birth and at 36 weeks PMA were calculated using the INTERGROWTH-21st growth curves. The primary outcome was cognitive delay at 2 years of age (Bayley-III cognitive score < 85). Results: Neither the traditional definition of PGF (z score below −1.3) nor the recently proposed definition of malnutrition (z score decline of 1.2 or greater) was associated with cognitive delay. Both a weight z score below −1.0 at 36 weeks PMA (RR: 1.65; 95% CI: 1.10–2.49; p < 0.05) and a decline below −1.0 in weight z score from birth to 36 weeks PMA (RR: 1.40; 95% CI: 1.00–1.94; p < 0.05) were associated with a higher risk of cognitive delay. Conclusion: With optimal cutoffs, INTERGROWTH-21st weight z scores can predict the risk of cognitive delay. Impact: New growth curves generated with longitudinal data could overcome some limitations of traditional growth curves generated with cross-sectional data. When these new growth curves are used to assess the growth of preterm infants, alternative definitions for postnatal growth alterations may be needed.This study examines the association between postnatal growth alterations defined by the INTEGROWTH-21st growth curves and adverse cognitive outcomes at 2 years of age.With alternative definitions of postnatal growth failure and malnutrition, the INTERGROWTH-21st growth curves can help establish the association between postnatal growth of extremely preterm infants and adverse neurodevelopmental outcomes in early childhood.