ABSTRACT
Throughout normal gestation, the placenta functions as the fetal hemodialyser, and the fetal kidneys play a minor role in the maintenance of fetal salt and water homeostasis. Formation of urine begins between the fifth and ninth weeks of gestation. The rate of urine production increases throughout gestation and, at term, urine output is 28-50 ml/h (Fig. 85.1).6,7 Normally, fetal urine is hypotonic.8 The glomerular filtration rate (GFR)
has been measured at 6 ml/min/1.73 m2 at 28 weeks’ gestation, increasing to 25 ml/min/1.73 m2 at term, and thereafter triples by 3 months of age. The main factors responsible for this rise in GFR include an increase in the capillary surface area available for filtration, changes in intrarenal vascular resistance, and redistribution of renal blood flow to the cortical nephrons, in which the majority of nephrons are located.9 A congenital obstructive lesion of the urinary tract may have a deleterious effect on renal function.
