BACKGROUND: Small for gestational age is a complex perinatal syndrome associated with increased neonatal morbidity, mortality, and impaired childhood growth and neurodevelopment. Current classifications rely primarily on birthweight, which does not capture the heterogeneity of the condition nor predict long-term health outcomes. Here we aim to identify and characterize distinct small for gestational age subgroups and assess their neonatal and early childhood health trajectories. OBJECTIVE: To refine the classification of small for gestational age by identifying subgroups based on maternal, fetal, and environmental factors and evaluating their associations with neonatal morbidity, growth, and neurodevelopment at age 2. STUDY DESIGN: Prospective cohort study. In six countries worldwide, between 2012 and 2018, the INTERBIO-21st Study enrolled small for gestational age and non-small for gestational age newborns defined by the <10th centile of international standards with moderate (≥third-<10th centile) and severe (