Q: What are the possible causes of fetal hydronephrosis?

A: Possible causes of prenatal hydronephrosis include:

· No significant abnormality: many of these dilated kidneys prove to be normal after delivery.

· Blockage: obstruction of the ureter can happen at the bladder in the ureterovesical junction (UVJ), or in the urethra at the posterior urethral valve (PUV).

· Urine Reflux: vesicoureteral reflux occurs when the valve between the bladder and the ureter does not function properly, permitting urine to flow back up to the kidney when the bladder fills or empties. Most children (75%) outgrow this during childhood but need daily antibiotic prophylaxis to prevent kidney damage before then.

· Duplications: perhaps 1% of all humans have two collecting tubes from a kidney. These may show up on fetal ultrasound. Occasionally, patients with duplication have a ureterocele, which is a balloon-like obstruction at the end of one of the duplex tubes.

· Multicystic kidney: this is a nonfunctional, cystic kidney.

The majority of cases (about two-thirds) of hydronephrosis are attributed to ureteropelvic junction (UPJ) obstruction, the rest are secondary to vesicoureteral reflux, or other causes.