The prevention of postnatal growth failure in very low birth weight (VLBW) preterm infants is of utmost importance. Standard fortification is the most commonly used supplementation practice but it does not consider the native variability of human milk. Data on efficacy and safety of prolonged target fortification are scarce. The investigators performed a prospective interventional study in VLBW preterm infants, exclusively fed with human milk, to test efficacy and metabolic safety of target fortification in these preterm infants.