The primary aim of this research is to determine whether supplementation with probiotics during the first weeks of life reduces the risk of necrotizing enterocolitis (NEC) and neonatal mortality and is safe to use among extremely preterm (EPT) infants born before gestational week 28. P: The study population include EPT infants (n= 1620) born at six tertiary neonatal units in Sweden and four units in Denmark. I: This is a double-blinded multicenter randomized controlled trial where infants in the intervention group will as soon as they tolerate 3 mL breastmilk per feed receive a probiotic combination of Bifidobacterium infantis, Bifidobacterium lactis, and Streptococcus thermophilus diluted in 3 mL breastmilk and given once daily until gestational week 34. C: The control group will receive 3 mL breastmilk without probiotic supplementation (blinded) daily. O: Primary outcome variables is a composite endpoint of incidence of NEC and mortality. Secondary outcomes include incidence of sepsis, duration of hospital stay, use of antibiotics, feeding tolerance, growth, and body composition after hospital discharge. Patient benefit: To provide evidence on the usage of probiotics among EPT infants that are not currently covered by clinical recommendations. As EPT infants have the highest risk for NEC and mortality our results have the potential to change current recommendations and improve patient outcomes, decrease mortality, shorten hospitalization, and decrease overall health-care costs.