Parents should be advised to feed their babies breast milk or infant formula exclusively for the first 4 months of life. They should be specifically advised not to give free water to their babies. Newborns do not have the renal concentrating ability that they will have later in life, and can suffer from hyponatremia if free water is given.

The newborn kidney has a lower glomerular filtration rate (GFR) and, therefore, a lesser ability to concentrate urine than the mature kidney does. At birth, the serum creatinine level is reflective of the mother’s serum creatinine. This level then falls over a period of 7 to 10 days by about 50%, to a new level of approximately 0.5 mg/dL. The serum creatinine then continues to drop at a slower rate until 4 to 6 weeks of age, when the level is about 0.3 mg/dL. Although muscle mass increases with growth, the creatinine level remains stable during the first 2 years of a child’s life. This is because of an increased GFR, which causes the creatinine level to stay the same despite an increased muscle mass. Beyond 2 or 3 years of age, any further increase in muscle mass is reflected by a rise in serum creatinine concentration.