What is the significance of finding dilated bilateral renal pelves on routine second trimester Ultrasound?
Prior to 23 weeks gestation, the normal diameter of fetal renal pelves is ≤5 mm, and after 23 weeks, ≤7 mm. Dilated renal pelves can be the result of vesicoureteral reflux, ureteropelvic junction obstruction, posterior urethral valves (where the bladder would also be distended), duplicated renal collecting system, or ureterocele. Pyelectasis is also seen in up to 25% of fetuses with Down syndrome. If dilation of the renal pelves (with or without caliectasis) is detected in utero, postnatal follow-up US is required to assess the degree of any remaining obstruction and renal function.