Antenatal (before birth) hydronephrosis (fluid-filled enlargement of the kidney) can be detected in the fetus by ultrasound studies performed as early as the first trimester of pregnancy. In most instances this diagnosis will not change obstetric care, but will require careful follow-up and possible surgery during infancy and childhood.

What causes antenatal hydronephrosis?

Possible causes of antenatal hydronephrosis include:

• Blockage: this may occur at the kidney in the ureteropelvic junction (UPJ), at the bladder in the ureterovesical junction, or in the urethra (posterior urethral valve).
• Reflux: vesicoureteral reflux occurs when the valve between the bladder and the ureter does not fuction 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 try to prevent kidney damage before they outgrow the reflux.
• 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.
• No significant abnormality: many of these dilated kidneys prove to be normal after delivery.

http://www.pedisurg.com/PtEduc/Antenatal_Hydronephrosis.htm

I agree with your doctor that there is nothing to worry about because this is a common finding at the 20 week ultrasound. There will most likely be another ultrasound later, which will show everything is fine.

Hope that helps,

Please let me know if you have more questions.

Kerry