Fluid management of the pediatric surgery patient is a critical element of care. Infants and children are sensitive to small degrees of dehydration, and commonly used protocols for pediatric fluid therapy do not consider rapidly changing perioperative physiology. The total body water content of a term gestation newborn is 75-80%. Total body water decreases by 4-5% during the first week of life and is reflected in weight loss. By age 1 year, total body water slowly decreases to the adult levels of 60%. Extracellular water content decreases in parallel with total body water content from 45% at term to 20-25% of adult levels by age 1 year. 
For a premature neonate, both total body water and extracellular fluid (ECF) increase with decreasing gestational age. For example, a premature neonate's extracellular water content at 28-32 weeks' gestation is 52% of his or her body weight. By age 1 week, the proportion of extracellular water decreases 12%; neonate unloads in 1 week of life what would have taken 8 weeks in utero.
Smith & Nephew