Inclusion Criteria:  All GA<37 weeks preterm infants with birth weight (BW) between 500 and 1500 grams born in Linkou of Chung Gung Memorial Hospital from August 2020 to July 2023 will be enrolled into study after obtaining informed consent from their parent within 1-week-of-age.  Exclusion Criteria:  Preterm infants who has major or congenital gastrointestional (GI) anomaly (eg. trisomy 13, trisomy 18, esophageal or intestinal atresia, GI obstruction, meconium peritonitis with prenatal bowel perforation, etc.).