I don't know of any babies that have only had a cyst on one kidney. I know of several cases of kidney cysts before birth but it has always affected both kidney's not just the one. If it is only one does that mean the second kidney is developing and working normally? If so then although it is a big concern there are a number of people who are only born with one kidney or loose one for a number of reasons and lead completely normal lives with no consequences to having only the one. And that would be looking at the worst case scenario of loosing that kidney. If there is just a blockage then they can dilate the tube to improve its functioning. I guess its on of those things you wont know the severity of until after bub is born and they look at urine function and do further scans.