This is a randomised controlled multi-centre trial comparing the effect of diet supplementation of a human breast milk-based nutrient fortifier (H2MF®) with standard bovine protein-based nutrient fortifier in 222 extremely preterm infants (born before gestational week 28+0) exclusively fed with human breast milk (own mother´s milk and/or donor milk). The infants will be randomised to receive either the human breast-milk based H2MF® or the standard bovine protein-based nutrient fortifier when oral feeds have reached <100 ml/kg/day.

The randomised intervention, stratified by centre, will continue until the target gestational week 34+0. The infant must not be fed with formula during the intervention period. The allocation will be concealed before inclusion, but after randomisation the study is not blinded.

Primary endpoint of the intervention is the composite variable necrotizing enterocolitis (NEC), sepsis and mortality.

The enrolled infants are characterised with clinical data including growth, feeding intolerance, use of enteral and parenteral nutrition, treatment, antibiotics and complications collected daily in a study specific case report form from birth until discharge from the hospital (not longer than gestational week 44+0). A follow up focusing on neurological development, growth and feeding problems will be performed at 2 years of age (corrected) and 5.5 years of age.

This is a randomised controlled multi-centre trial comparing the effect of diet supplementation of a human breast milk-based nutrient fortifier (H2MF®) with standard bovine protein-based nutrient fortifier in 222 extremely preterm infants (born before gestational week 28+0) exclusively fed with human breast milk (own mother´s milk and/or donor milk). The infants will be randomised to receive either the human breast-milk based H2MF® or the standard bovine protein-based nutrient fortifier when oral feeds have reached <100 ml/kg/day. If fortification with extra enteral lipids is needed during the intervention period, the infants receiving H2MF® will be supplemented with the human milk-based Prolact CR®, while the infants receiving standard bovine protein-based fortification will be supplemented with the standard lipid products used at the unit. The study subject will be enrolled at level III neonatal intensive care unit (NICU)s. Only infants with a home clinic with the logistics to maintain the intervention until gestational week 34+0 will be included.

The randomised intervention, stratified by centre, will continue until the target gestational week 34+0. The infant must not be fed with formula during the intervention period. The allocation will be concealed before inclusion, but after randomisation the study is not blinded. It would not be possible to prescribe the fortifier and prepare of the breast milk in a blinded fashion, since the fortifiers are not exactly equal in nutrient content and also look different. Instead the assessment of several of the outcomes will be made blinded, such as the assessment of X-ray images in NEC cases.

The enrolled infants are characterised with clinical data including growth, feeding intolerance, use of enteral and parenteral nutrition, treatment, antibiotics and complications collected daily in a study specific case report form from birth until discharge from the hospital (not longer than gestational week 44+0). A follow up focusing on neurological development, growth and feeding problems will be performed at 2 years of age (corrected) and 5.5 years.

Since it is often difficult to distinguish between the diagnoses of NEC and sepsis, and their clinical consequences, the investigator's primary endpoint of the intervention is the composite variable NEC, sepsis and mortality. Secondary endpoints are feeding intolerance and other severe complication such as Bronchopulmonary dysplasia (BPD), Retinopathy of prematurity (ROP) and neurological impairment. Stool, urine and blood samples are also collected for microbiology, metabolomic and immunology analysis in order to study underlying mechanisms. Health economic analyses will be made to evaluate the costs and benefits of an introduction of human milk-based fortifier in NICUs in the Nordic countries.

Analyses will be conducted using an intention to treat approach. An evaluation will be performed when 20 infants have been included to evaluate feasibility and make it possible to adjust the protocol for the remaining part of the study. Safety analyses will be performed by an independent data and safety monitoring board (DSMB) when 50, 100 and 150 infants have been included. A sample size re-estimation will be made by an independent statistician when 150 infants have been included. Thus, the definitive sample size might be increased (never decreased) based on this interim analysis. The study can be terminated before 322 infants have been enrolled based on a decision of the sponsor and the DSMB, if the primary outcome is significantly lower (with a significance level <0.001) in the H2MF® than in the standard fortification group in the interim analysis made after 150 infants have completed the neonatal period. The study subject will be enrolled at level III NICUs in the Nordic Countries. All study subjects will be followed during the neonatal period until discharge (not longer than gestational week 44+0) and also be included in a follow up at 2 and 5.5 years of age based on the national follow up program for extremely preterm infants.

Inclusion Criteria:

• Gestational age at birth 22+0-27+6: based on prenatal ultrasonography.
• Enteral feeds < 100 mL/kg/day at the day of randomisation.
• Written informed consent from the legal guardians of the infant.
• The home clinic of the infant has the logistics of maintaining the intervention until gestational week 34+0

Exclusion Criteria:

• Lethal or complicated malformation known at the time of inclusion
• Chromosomal anomalies known at the time of inclusion
• No realistic hope for survival at the time of inclusion
• Gastrointestinal malformation known at the time of inclusion
• Abdominal surgery before the time of inclusion
• Participation in another intervention trial aiming at having an effect on growth, nutrition, feeding intolerance or severe complications such as NEC and sepsis
• Infants having nutrient fortifier or formula prior to randomisation