Changes occur in the total body water volume, extracellular fluid volume, and intracellular fluid volume during the transition from fetal to postnatal life. Early in gestation the fetus is composedalmost entirely of water and at term is 73% fluid, compared with 58% in the adult. The fetus has more extracellular fluid than intracellular fluid, but this shifts progressively throughout postnatal life, probably because of the growth of cells at the expense of extracellular fluid. The infant has a proportionately higher ratio of extracellular fluid than the adult and consequently has a higher level of total body sodium and chloride and a lower level of potassium, magnesium, and phosphate.