Extra-uterine growth restriction (EUGR) is one of the most common findings among very preterm infants at discharge from neonatal intensive care units. EUGR is associated with major morbidities such as bronchopulmonary dysplasia (BPD), retinopathy of prematurity (ROP) and impaired neurodevelopment. EUGR is caused by slow postnatal growth and largely results from energy and protein deficits. These deficits occur despite the current fortifications of human milk. Hindmilk, the milk at the end of a breast pumping session, has higher fat and energy content compared to the composite milk. Feeding hindmilk can be a natural and innovative way to provide additional calories for very preterm infant. Hindmilk is rich in some fatty acids that are currently under investigation to prevent BPD and ROP and improve neurodevelopment. The aim of this study is to assess growth benefits of feeding hindmilk and to explore whether it can improve fatty acids profile in very preterm infants.

Dietitians (RD) and Lactation Consultant (LC) will screen and identify eligible mother-infant pairs. Once mother's consent obtained, LC will hand out the "hindmilk information sheet" and teach the mother how to separate her milk. A sample (10 ml) of composite pumped fresh milk will be collected and placed in the fridge. These samples will be sent for analysis within 24 hours of collection to minimize any changes to contents. Samples for Erythrocyte membrane fatty acid (FA) profile will be collected on dried blood spot (30-100 µL). These samples will be collected within 72 hours of the consent. It will be coordinated with blood tests ordered by the clinical team within that period. Dried blood spots are stable in room temperature for 28 days however we will store them in the -80̊ C freezer within 72 hours of collection. Another milk sample, hindmilk this time, will be sent for testing within 24-48 hour of starting hindmilk. This is to ensure that assessment of milk contents is performed in the same mother's lactation stage. The second dried blood spot will be drawn after 2-4 weeks of starting hindmilk. The 2-4 weeks period is to allow timing with other blood work-ups. In General, blood glucose is done every 2-3 days in babies with poor growth and growth laboratory testing is normally done every 2-3 weeks.

Data on fluid volumes, feeds, macronutrients intakes and any change in nutrition plan will be collected from electronic dietitian's notes. Dietitians will ensure proper and detailed documentation of nutritional information. Average weight gain (in gram/kg per day) is calculated as mentioned before. Average weight gain will be compared initially between the week before and the week after starting hindmilk. The day that hindmilk started will be used to identify the start point however it will not be used in either the pre- or the post-hindmilk calculation. Clinical team will be encouraged to not order other changes to the nutrition plan for the first week after starting hindmilk. Daily weight, weekly length and head circumference will be collected from the electronic charts. Furthermore, weight, length and head circumference at completed gestational week will be used to calculate Z scores using Fenton Z scores calculator.

Data on maternal and neonatal characteristics will be collected from their electronic and physical charts. Composite milk, foremilk and hindmilk volumes will be collected from the collection sheet that has been already used in our NICU.