Therefore, the overarching aim of the PreMiBraIn study is to elucidate the role of the gut-immune-brain axis on neonatal brain injury and its impact on long-term neurodevelopmental outcome of extremely premature infants. The study cohort will consist of a total of 60 extremely premature infants with a gestational age < 28 weeks and birth weight < 1000 grams. The investigators seek to characterize the orchestrated dynamics of the maturation of the gut microbiome and the subsequent impact on maturation of innate and adaptive immune mechanisms as well as neurophysiological maturation and neurodevelopmental outcome. Furthermore, the investigators will assess the value of the microbiome as a prognostic indicator for neonatal brain injury as well as short- and long-term neurodevelopmental outcome of extremely premature infants.