Study groups: 1, exclusive breast milk; 2, mixed feeding; 3, exclusive artificial feeding. ΔZ-Score: mean difference in Z-score for weight, length, and head circumference between discharge and birth. Linear regression model adjusted for bronchopulmonary dysplasia, periintraventricular hemorrhage, SNAPPE II, weight and gestational age at birth, occurrence of enterocolitis and sepsis. *P<0.05.