Hi, My eldest daughter was born with only one kidney. They picked it up on an ultrasound and confirmed it when she was 6 weeks old (by ultrasound). We had check ups for a while but they stopped when she was 18months old as all was fine. She just has to get her blood pressure checked yearly and if we have any unexplained fever (i.e. not from a cold or bug) we need to check her urine for a UTI.
However she has only ever had 1 uti and is perfectly healthy. Apparently 1 in 800 babies are born minus a kidney. Her right kidney is a little larger than normal, maybe to compensate. It really hasn't been anything to worry about though. I did for about the first year though