Nephrons are not functionally mature in the neonate. The formation of new nephrons continues at least 3 weeks post partum in dogs. There is a gradual transition from neonatal to adult metabolism. The ages listed below are not absolute, but are guidelines.
Tubular immaturity exists.
Fluid intake and excretion of fluids are 5 - 7 times as great in relationship to weight compared to adults making the neonate more susceptible to fluid depletion. This needs to be taken into consideration when formulating a fluid therapy plan for a neonatal animal. Until 8 weeks of age the urine specific gravity is in the range of 1.006 - 1.017. Adult animals often have a urine specific gravitiy in excess of 1.035. Because of renal immaturity and high water turnover, neonates are very susceptible to fluid imbalance and intoxication with renally excreted substances.