This study is a randomized trial comparing 2 methods of human milk fortification for preterm infants in the neonatal intensive care unit (NICU). All participating infants will receive a human milk diet comprising maternal and/or donor milk plus multi-component and modular fortifiers. In one group (control), the milk will be fortified according to routine standard of care. In the other group (intervention), the fortification will be individually targeted based on the results of point-of-care human milk analysis. Outcomes include physical growth in the NICU and after discharge, brain structure by magnetic resonance imaging at term equivalent age, and neurodevelopment at 2 years.

Human milk is the recommended diet for preterm infants and it requires fortification to meet their high nutrient requirements. This study addresses a gap in knowledge about the optimal strategy for human milk fortification in the neonatal intensive care unit (NICU).

Infants will be recruited from a single site (Brigham and Women's Hospital, Boston MA). Eligible infants are NICU inpatients born <31 weeks' gestation and enrolled within 21 days of birth, whose mothers are providing breast milk. Exclusion criteria include major congenital anomalies, severe fetal growth restriction (weight <3rd percentile), clinically significant gastrointestinal pathology, triplets or higher order multiples, and expected death or transfer. Infants who require fluid restriction <140 mL/kg/day will also be excluded.

The study period begins when infants have reached full volume (150-160 mL/kg/day) human milk feedings fortified to 24 kcal per ounce. Participants will be randomized to one of two diet groups; twins will be randomized separately. The study diet will continue until 36 weeks' postmenstrual age or hospital discharge, whichever comes first.

Available maternal milk will be pooled with donor milk (if needed) to reach the total daily fluid volume; this "base" milk will be analyzed with a point-of-care human milk analyzer (Miris, AB). All infants will receive at minimum the standard human milk fortification (multicomponent fortifier to 24 kcal/oz plus liquid protein 0.27 g/dL). In one group (control), we will follow the standard of care for milk fortification, only adding more protein and/or energy if weight gain is lagging. In the other group (intervention), we will individually target fortification with additional protein and/or energy so that the "base" milk always contains protein 1 g/dL and energy 67 kcal/dL.

Outcome measures will include physical growth during the diet intervention period, including gain in weight, length, and head circumference and body composition with air displacement plethysmography at the end of the diet intervention. At term equivalent age, participants will undergo brain magnetic resonance imaging. At 2 years corrected age, outcomes include standard and novel neurodevelopmental tests including the Bayley-III and tests of emerging executive function; and physical size.

Inclusion Criteria:

• Inpatient in Brigham and Women's Hospital NICU
• Gestational age 24 0/7 to 30 6/7 weeks
• Chronologic age <21 days
• Mother providing breast milk

Exclusion Criteria:

• Major congenital anomaly
• Severe fetal growth restriction (birth weight <3rd percentile by Olsen reference)
• Necrotizing enterocolitis, intestinal perforation, other major gastrointestinal pathology
• Triplets or higher order multiples
• Plan for redirection of care and/or anticipated death
• Clinically significant renal or hepatic dysfunction
• Inborn error of metabolism
• Fluid restriction <140 mL/kg/day for 3 or more days
• Grade 3 or 4 intraventricular hemorrhage detected prior to enrollment
• Anticipated transfer <36 weeks' postmenstrual age
• Parents do not consent to use of pasteurized donor human milk
• Infant in non-parental custody

No current plans to share individual patient data. Future data sharing would involve de-identified data only and would require a data use agreement in accordance with hospital policy.

Parents, clinical, and research staff apart from the study milk technician will be unaware of the treatment group.