Preterm infants compared to the term ones have proportionally more fluid in the extracellular compartment than in the intracellular and are thus susceptible to free water loss. Postnatal weight loss is up to 15 – 20% of birthweight compared to 10% in term infants. Disturbances of fluid balance contribute to intraventricular hemorrhage and patent ductus arteriosus. Nonoliguric hyperkalemia, hyponatremia or hypernatremia are frequent problems.