The proposed research will investigate individualized fortification of breast milk based on daily milk analysis of carbohydrate, protein, and fat content in a randomized double blind controlled trial. The combination of additional fat, carbohydrate and protein and commercial fortifier will be added to ensure that the milk contains the target amounts of nutrient. Growth and development of these infants will be compared with that of infants fed mother's milk that has been supplemented with the current standard amounts. The postnatal growth of the infants will be assessed by measuring weight, length and head circumference and fat and lean mass using highly accurate, non-invasive methods throughout the intervention period and at the first follow-up visit after discharge at 3 months. Neurological development will be analyzed at the age of 18 months.

The investigators hypothesize that individualized fortification of breast milk improves the nutritional intake of preterm infants, optimizing growth, and thus this will positively impact neurodevelopment and health.

Inclusion criteria:

1. Gestational age < 32weeks (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; and
4. Written informed consent has been obtained from the infant's legal representative.

Exclusion criteria:

1. Infants with intrauterine growth restriction, small for gestational age defined by a weight less than 3rd percentile using sex specific reference data for birth weight
2. Gastrointestinal malformation, major congenital anomalies and chromosomal abnormalities;
3. Babies with enterostoma or short gut syndrome;
4. 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);
5. Renal disease, defined by symptoms (oliguria, anuria, proteinuria, hematuria) associated with an increased blood urea nitrogen 10 mmol/L79 and creatinine of 130 mmol/L80;
6. Hepatic dysfunction, defined by jaundice (direct bilirubin >1.0 mg/dl) that is associated with one or more abnormal liver function tests (AST, ALT or GGT);
7. Participation in another clinical trial that may affect outcomes of this study; or
8. Probability of transfer to another NICU or level II nursery outside the McMaster Children's Hospital before the minimum period of three weeks is completed.

Post-randomisation exclusion criteria:

1. Infants fed more than 25% of mean caloric intake for a consecutive week with formula milk;
2. Fluid restriction < 140mL/kg/d for ≥ 3 consecutive days;
3. Sepsis - all infants with gram-negative sepsis will be removed from the study.