Anyone ever had anything like an enlarged kidney show up on the ultrasound?

I had my 20 week ultrasound for our baby girl on Monday afternoon, I just got a call from the doctors office saying one of her kidneys is enlarged and they want to do another ultrasound at 33 weeks to see if the problem corrects itself. She said apparently it is command I shouldn't worry of lose any sleep over it but how am I not supposed to do that? I never had anything like this with either of my two boys so it's something I am worrying about...