I was born with just one kidney. I am missing my kidney on my right side. I didn't find out I only had one, until I was 25. Most of the time, when a baby is being formed in utero and a kidney is missing, the one kidney that is there, ends up being twice the size as a normal kidney, it's the body's way of compensating for the other kidney not being there. That's the case with mine, it's double in size, and it functions just fine.