Serum and urine osmolality in normal full-term infants.

Serum and urine osmolalities were measured for the first five days of life in healthy fullterm newborns fed from the first day. Values of serum osmolality were found to be lower than in previous studies of newborns and lower than those of children and adults. The early initiation of feeding, the low solute concentration of the formula and the relatively low glomerular filtration rate of newborns may account for these observations. A rise in serum osmolality concurrent with the elaboration of a more dilute urine suggests that maturation of mechanisms involved in water excretion occurs in the first five days of life.