How do you tell when you should go up a size? I knew he needed out of newborn sizes because he was leaking and they were starting to be tight in his legs but when should we go to size 2? They fit his belly area but his legs are starting to get marks but not bad. He's only just over 6 weeks but weighs 11.5 pounds during last appointment. I'm worried size two will be to big on his tummy even tightened as far as they can go.