My son has this. Discovered at 24 weeks and my OB said it usually goes down on it's on. It didn't and was still present at birth (36 weeks) so we were sent to a urologist at a week old. The doctor was wonderful! They had to do an x-ray and catheter (which was the worst part to me) to get a better view and the doctor said it's a minor thing and usually develops with age. He went for a check up at 6 months and it still hadn't resolved completely, but was on the right track. He goes back for another check up in April.
I was SO worried because I saw it could be a marker for Downs, but when I asked, they said it's only considered a marker for Downs if there are at least 3 other markers present. The dilated kidney in itself if not a marker for Downs.