Borderline genitourinary tract abnormalities.

The ability to detect major malformations of the fetal urinary tract using diagnostic antenatal ultrasound is well-known. Serious obstructive or dysgenetic abnormalities are often clearly seen and their implications well-known. Less clear is the meaning of mild or minimal pyelectasis, unilateral renal agenesis, nonvisualization of the fetal bladder, and pelvic kidneys. Fetal pyelectasis rarely, if ever, is associated with a need for neonatal intervention if the diameter of the pelvis is less than 10 mm. Although mild pyelectasis occurs in up to 25% of Down syndrome fetuses, the risk of Down syndrome in the case of isolated pyelectasis in a low-risk population is less than 1 in 340. Mild pyelectasis by itself does not seem to justify invasive testing for fetal karyotype. Nonvisualization of the fetal bladder carries a high risk of renal agenesis or dysplasia, whereas unilateral renal agenesis and a fetal pelvic kidney seem to carry limited, if any, negative implications.