Target Fortification of Breast Milk: How Often Breast Milk Needs to be Measured?

Inclusion Criteria: 1. Gestational age \< 30 weeks (maternal dates or early fetal ultrasound); 2. Tolerating an enteral intake of ≥100 mL/kg/d for ≥ 24h; 3. Subject is anticipated to receive the intervention for ≥ 3 consecutive weeks after full enteral feeding (≥150 mL/kg/d) has been achieved; 4. Written informed consent has been obtained from the infant's legal representative. 5. Multiple births: Each infant will be included in the study if he/she meets the study criteria, and siblings will be individually randomized to one or other of the treatment arms. 6. Discussion with Most Responsible Physician (MRP) and the staff in order to discuss any potential transfer during the next 7 days. Exclusion Criteria: 1. Gastrointestinal malformation, major congenital anomalies and chromosomal abnormalities; 2. Babies with enterostoma or short gut syndrome; 3. Infants fed more than 25% of mean caloric intake for a consecutive week with formula milk; 4. Fluid restriction \<140 mL/kg/d for ≥ 3 consecutive days; 5. Sepsis - all infants with gram-negative sepsis will be removed from the study; 6. Necrotizing enterocolitis, defined by feeding intolerance associated with positive x-ray findings (pneumatosis intestinalis - Bell Stage 2; air in the biliary tract or free air in the peritoneum - Bell Stage 3); 7. Renal disease, defined by symptoms (oliguria, anuria, proteinuria, hematuria) associated with an increased blood urea nitrogen \>10 mmol/L and creatinine of 130mmol/L 8. Participation in another clinical trial that may provide an alternative nutritional intervention, which might affect the outcomes of this study. outcomes of this study; 9. Probability of transfer to another neonatal intensive care unit or level II nursery outside the McMaster Children's Hospital, as discussed with the most responsible physician (MRP)