A loose rule for a good height/weight balance until around 50 inches is 1 lb per inch. (after 50 inches doc wants to see them add weight at a higher rate) This is not to say that if your child is 45 inches and 38 lbs there is a problem - only your doctor can tell you that, but your son sounds like he is within normal limits"For what it is worth, my 3 y/o is 39 inches and 40 lbs - solid, but in no