This study is a randomized controlled trial comparing standard fortification of donor breast milk to targeted fortification of donor breast milk in preterm infants. The purpose of the study is to determine if there is a benefit to target fortifying donor breast milk in the preterm population. The investigators hypothesize that infants receiving targeted fortification of donor breast milk will have improved growth compared to infants receiving standard fortification of donor breast milk.

Breast milk is the best food for premature infants due to its ability to protect infants from necrotizing enterocolitis (NEC) and late onset sepsis. When a premature infant's mother's own milk supply is not enough to provide all the milk that her premature infant needs, donated breast milk is the next best option. One of the downsides of donor breast milk is that it often does not contain as much nutrition for the developing infant compared to the milk of a mother of a premature infant. This means a higher likelihood for poor growth in infant receiving mostly donor breast milk compared to mom's own milk. Due to an inability to easily measure the nutritional content of donor breast milk, standard practice has been to assume that the milk has a certain amount of fat, carbohydrates, and protein. Based on these assumptions, a set amount of additional nutritional fortifier is added to both donor and mom's own breast milk prior to it being given to the infant. Studies show that there is often less nutrition in the donor breast milk to start with than assumed. Technology is now available which uses a small volume sample to measure fat, protein, and carbohydrates in human milk, and hence, calculate the calories in the milk. Using this technology, in this study, the investigators will customize the fortification of donor breast milk by first measuring what is in the milk and then adding any additional fortification that is needed to reach the recommended goals for fat, protein, and carbohydrates. The investigators hypothesize that infants receiving this customized milk, so-called "target" fortified donor breast milk, will have better growth than infants receiving the standard amount of fortification added to their donor breast milk.

The study design involves measurement of macronutrient and calorie content of donor breast milk using a point-of-care human milk analyzer (Miris, AB). The individual macronutrient (protein, fat, and carbohydrate) concentrations will be targeted in the intervention group such that the infant will receive protein of 4 g/kg/day, fat of 6.6 g/kg/day, and carbohydrates of 11.6-13.2 g/kg/day. Fortification will be added in a step-wise fashion daily over a 3-4 day period starting when the infant reaches about 80 ml/kg/day in feeding volume. Fortifiers that may be used in this study include: Abbott Similac Human Milk Fortifier Extensively Hydrolyzed Protein Concentrated Liquid, Abbott Liquid Protein, Nestle Microlipid, and Medica Nutrition SolCarb. Participants in the control arm will receive donor milk that is fortified in the standard fashion at this institution; that is, 4 packets of Human Milk Fortifier will be added to 100 ml of donor breast milk. Additional fortification such as liquid protein or microlipid may be added to the donor milk and mom's own milk in response to poor growth for participants in both groups, as is standard of care in this Neonatal Intensive Care Unit (NICU). Patients in both experimental and control groups will always receive mom's own milk first when it is available. Mom's own milk will not be analyzed and will always be fortified in the standard fashion for both groups. The participants will be studied until they reach 34 weeks corrected gestational age.

Inclusion Criteria:

• Premature infants born \

Individual participant data (IPD) may be shared with other researchers at Columbia University Medical Center at the end of the study for the purposes of creating a neurodevelopmental follow-up study that involves following participants in the study until they reach 2 years (corrected post-natal age) in neurodevelopment follow-up clinic. Permission for future contact is included in the consent form for participants in this study. Only participants who consent to future contact will be approached about a neurodevelopmental follow-up study.