This is picked up a lot on u/s before birth, but some appear to resolve in the first 2 years of life. They will keep close tabs on bub during childhood to make sure this is what occurs.

Sometimes the obstruction remains, causing significant dilation and usually accompanied by pain - usually again picked up in childhood but rarely in adulthood. I can speak with authority on this as only a year ago my husband had some odd pain, and following an u/s and CT, it was an obstructed and badly dilated kidney (more than twice normal size). He's had surgery now to try to rectify it with limited success. The success rate for surgery though is 95-98% so he's just unlucky. The surgeon in Melbourne says he does about 2 per year, so relatively rare. Don't stress about it, chances are it will go away with no lasting probs.