Fig. 1: Inclusion flow chart.

VLBW very-low-birth-weight; KNN Korean Neonatal Network; NEC necrotizing enterocolitis; Control group, EPIs without NEC; mNEC group, EPIs with medically treated NEC; sNEC group, EPIs with severe NEC that was treated by surgical intervention including peritoneal drainage or laparotomy, or who died before surgery with clinical evidence of requiring surgery.