I was a little hesitant at first to order this from reading other reviews on Mudpie clothing sizes running small, but I decided to go ahead and give it a shot, and it fits my son perfectly! I ordered the size 9-12 months. My son will be one day shy of 10 months come Christmas time. I only wish the sweater was thicker. For the price I expected something more. It feels like a t-shirt. I guess that will work out okay though since my son tends to sweat a lot.