The purpose of this two-arm investigation is to determine if growth patterns of very low birth weight infants (VLBW) (birth weight 750-1500 grams) fed human milk (maternal or donor) supplemented with a human milk-based fortifier grow according to established guidelines and maintain adequate micronutrient levels.

To achieve this goal, the investigators will prospectively analyze the growth and micronutrient status of VLBW infants who are fed human milk (maternal or donor) supplemented with a human-milk-based fortifier with increased protein (Medolac® Human Milk Fortifier). In addition, the investigators will compare the findings to retrospectively collected data for growth rates and micronutrient status of infants who received human milk fortified with cow's milk -based fortifier (Enfamil® Hydrolyzed Liquid Human Milk Fortifier). The investigators hypothesize that a human milk-based fortifier with increased protein will support growth at recommended levels (weight gain of 12-18 g/kg/day, head circumference 0.75-1.0 cm/week, length 0.8-1.1 cm/week)[1-3] and prevent micronutrient deficiency in the VLBW infant

Aim 1: To determine if VLBW infants fed human milk, maternal or donor, supplemented with a human milk-based fortifier with increased protein grow at recommended levels for weight, length, and head circumference. To achieve this aim, Z-scores for weight, length, and head circumference will be tracked. Measurements will be taken at birth and then weekly until 36 weeks post-menstrual age (PMA) or discharge from the neonatal intensive care unit (NICU), whichever comes first. Aim 2: To measure nutritional status in VLBW premature infants fed human milk supplemented with a human milk-based fortifier with increased protein. To achieve this aim, serum magnesium, potassium, chloride, blood urea nitrogen (BUN), creatinine, sodium, calcium, phosphorus, CO2, Vitamin D 1, 25 (OH) 2D, parathyroid hormone (PTH), alkaline phosphatase, hemoglobin, hematocrit will be measured within 24 hours of reaching full enteral feedings and repeated seven days later, and then every fourteen days until 36 weeks PMA or discharge, whichever comes first. Urine magnesium and sodium will be measured on the same schedule.

Aim 3: To compare growth rates and nutritional status of VLBW infants fed human milk fortified with a human milk-based fortifier to growth rates and nutritional status of those fed human milk fortified with a cow's milk-based fortifier.

Inclusion Criteria:

1. Birth weight 750-1800 grams
2. Admitted to AU NICU within 24 hours of life
3. Estimated gestational age (EGA) 23 to 33 weeks as confirmed by the Ballard score
4. Birth weight appropriate for gestational age (AGA) defined as >3rd% on a gender-specific Fenton growth curve (Fenton 2013, Calgary, Canada)
5. Enteral feedings initiated within 7 days of life
6. Breastmilk diet, maternal or donor milk

Exclusion Criteria:

1. Renal conditions affecting electrolyte metabolism and/or excretion
2. Gastro-intestinal conditions that preclude feeding or affect nutrient absorption (gastroschisis, omphalocele)
3. EGA >33 weeks or birth weight >1800 grams or EGA <23 weeks or birth weight <750 grams
4. Apgar <3 at 5 minutes
5. Grade 3 or higher intraventricular hemorrhage (IVH)
6. Intrauterine growth restriction (IUGR), as defined as <3rd% on a gender-specific Fenton growth curve
7. Congenital anomalies including congenital heart disease or other major defect requiring surgical intervention
8. Intake of cow's milk formula or fortifier before or after the initiation of the study protocol