Our son was also a preemie - 21 inches long and only 4 lbs, 9 oz - you could see every rib on his body! HOwever, all also turned out well for us and he is a healthy 17 1/2 year old now. I'm not an expert, but myguess would be "average" is about 19 inches and 8 pounds. Any experts out there on these numbers?