Renal Agenesis / Hypoplasia

The kidneys begin to develop early in the 5th week of gestation and start to function about 3 weeks later. Formation of urine is thought to begin in the 9th to 12th week of gestation; by the 32nd week, the fetal production of urine reaches about 28 ml / hour. Urine is the main constituent of amniotic fluid. The relative amount of amniotic fluid can provide information about the status of fetal renal function.

The term agenesis refers to the absence of an organ because of failure to develop. Dys genesis is the failure to develop normally. Unilateral agenesis of the kidneys is relatively common, and many persons with this defect are unaware of it as long as the single kidney functions normally. 

Dysgenesis or agenesis of both kidneys is relatively rare; when either does occur, it often is accompanied by pulmonary hypoplasia. These defects are incompatible with life, and infants that have them are stillborn or die shortly after birth of pulmonary or renal complications. In renal hypoplasia, the kidneys do not develop to normal size. Like agenesis, hypoplasia more commonly affects only one kidney. 

When both kidneys are affected, there is progressive development of renal failure. In pregnancies that involve babies with nonfunctional kidneys or outflow obstruction of the kidneys, the amount of amniotic fluid is small a condition called oligohydramnios. The cause of fetal death in these babies is thought to be cord compression caused by the oligohydramnios.

Animal experiments have shown that ureteral or bladder outlet obstruction in the fetus causes renal dysgenesis or agenesis and pulmonary hypoplasia. Obstructions in fetal urinary outflow can be diagnosed with ultrasound imaging. In the normal fetus, the kidneys can be visualized as early as 12 weeks. The sensitivity with which fetal obstructive uropathy can be diagnosed is low before 20 weeks and high between 35 and 40 weeks. In utero surgery has been done to relieve outflow obstructions, with the hope of preventing pulmonary hypoplasia and renal dysgenesis. The procedure is experimental, and its success rate is still being determined.