The same thing happened to me! When I was at my 20 week ultrasound, they told me that my baby boy had an enlarged kidney, and that it's common in boys and should subside. They then scheduled me for a 28 week ultrasound (that I got today) and the kidney was still enlarged (one was 4.8mm and one was 6.0mm if I remember correctly). They said it shouldn't be anything to worry about, and in 4 weeks they scheduled another ultrasound to check up on it. The only thing that worries me about this was that an enlarged kidney in a fetus is a potential sign of Down Syndrome. This is rare and (through my research) an enlarged kidney is pretty common in most fetuses.