Fig. 1. Study population. Out of the cohort that included all preterm infants less than 34 weeks' gestation who were born and survived at the NICUs of 46 hospitals in Korea from April 2012 to September 2012 (n = 1,867), the infants who were followed up for > 1 yr after discharge from the NICU, or who had readmission after discharge from the NICU were enrolled in this study (n = 1,140).
