The widespread use and sensitivity of fetal ultrasonography has resulted in antenatal detection of the majority of renal malformations. Ultrasonographic screening for fetal anomalies can detect renal agenesis, multicystic dysplastic kidney, hydronephrosis, and abnormally shaped bladders from midway through gestation.
Fetal urinary production starts at 9 weeks gestation, therefore, the fetal bladder should be visualized from 13 weeks onward. By 20 weeks.fetal urine produces 90% of amniotic fluid. References curves for renal volume and the amount of amniotic fluid are available. Fetal kidneys can be visualized at 12 weeks, and by 25 weeks, the renal cortex and medulla are distincly demonstrated on ultrasonography. Parameters for expected length (appropriate growth) based on gestational age are also available.
Fetal hydronephrosis is most commonly detected during routine ultrasonography between 18 and 22 weeks gestation. Several systems has been developed to diagnose and grade the severity of hydronephrosis, but there is no consensus on the most appropriate criteria. In general, the likelihood of having a significant renal anomaly correlates with the severity of hydronephrosis. Because of renal immaturity, hydronephrosis detected before 25 weeks warrants repeat scanning. As expected, studies performed in the third semester are the most helpful in predicting the postnatal outcome of congenital anomalies of the kidneys and urinary tract. Thinning of the renal parenchyma and/or cortical cyst may be seen with hydronephrosis. They indicate injury or impaired development of the renal cortex. Increased echogenicity of the renal cortex may indicate anbormalrenal parenchymal development. These findings are associated with poor postnatal renal function when combined with hydronephrosis.
Normally, the fetal ureters are not seen on ultrasonography, so when they are visualized, ureteric or bladder obstruction or vesikooreteral reflux may be indicated. Bilateral involvement increases the risk of significant renal abnormality and of impaired postnatal renal function. The bladder wall is normally thin. If the bladder wal is thick, urethral obstruction such as posterior uretral valves in a male fetus may be present. If the bladder is not seen, consider the diagnosis of bladder extrophy.
Oligohydramnions at or beyond the 20th week of gestation is the most reliable predictor of abnormal fetal renal functions. Because amniotic fluid is predominantly composed of fetal urine, biochemical analysis is useful in further assesing fetal renal function. Sodium and chloride concentration greater than 90 mEq/L (90mmol/L) and urinary osmolality less than 210 mOsm/kg H2O (210 mmol/kg H2O) in the amniotic fluid are indicative of fetal renal tubular impairement and poor renal prognosis. Under these circumstance, more invasive testing such as vesicocentesis (bladder taps) may be undertaken.