An heterogeneous group of newborns with different gestational ages, birth weight, nutritional status and different risks of morbidity and mortality are classified as premature. Therefore the definition of premature children subgroups is mandatory. Very low weight premature babies are those with a birth weight of less than 1500 g. In Chile, they correspond to 0.9% of newborns and they contribute to neonatal mortality with 50 to 70% and to infantile mortality, with 25 to 30%. Survival of extremely premature babies has improved significantly in the last decades, due to a better perinatal care, the appearance of neonatal intensive care units, the routine use of mechanical ventilation, the perinatal use of steroids and the introduction of exogenous surfactant. However, survival varies considerably according to the place of birth, birth weight and gestational age and may be associated with different degrees of physical and mental disability. The imminent birth of a child in the limit of viability will require complex and difficult ethical decisions. There is consensus that the limit of viability is 23 weeks and that it fluctuates between 24 and 26 weeks in different parts of the world.