Ultrasound diagnosis of fetal renal abnormalities.

Development of the urogenital system in humans is a complex process; consequently, renal anomalies are among the most common congenital anomalies. The fetal urinary tract can be visualised ultrasonically from 11 weeks onwards, allowing recognition of megacystis at 11-14 weeks, which warrants comprehensive risk assessment of possible underlying chromosomal aneuploidy or obstructive uropathy. A mid-trimester anomaly scan enables detection of most renal anomalies with higher sensitivity. Bilateral renal agenesis can be confirmed ultrasonically, with empty renal fossae and absent bladder filling, along with severe oligohydramnios or anhydramnios. Dysplastic kidneys are recognised as they appear large, hyperechoic, and with or without cystic spaces, which occurs within the renal cortex. Presence of dilated ureters without obvious dilatation of the collecting system needs careful examination of the upper urinary tract to exclude duplex kidney system. Sonographically, it is also possible to differentiate between infantile type and adult type of polycystic kidney diseases, which are usually single gene disorders. Upper urinary tract dilatation is one of the most common abnormalities diagnosed prenatally. It is usually caused by transient urine flow impairment at the level of the pelvi-ureteric junction and vesico-ureteric junction, which improves with time in most cases. Fetal lower urinary tract obstruction is mainly caused by posterior urethral valves and urethral atresia. Thick bladder walls and a dilated posterior urethra (keyhole sign) are suggestive of posterior urethral valves. Prenatal ultrasounds cannot be used confidently to assess renal function. Liquor volume and echogenicity of renal parenchyma, however, can be used as a guide to indirectly assess the underlying renal reserve. Renal tract anomalies may be isolated but can also be associated with other congenital anomalies. Therefore, a thorough examination of the other systems is mandatory to exclude possible genetic disorders.