As the other reviewers mention, these onesies run very small, and Gerber's sizing information is not accurate.  My son is 13 months old and just starting to outgrow his 12-month-size clothes; he's lean but tall for his age (about 24 pounds, 31 inches).  Knowing Gerber runs small, I bought the 24-month size onesies, which ought to have been too big.  But Gerber's 24-month onesie is about the same size as my Carter's 12-month bodysuit.  The material is much thinner than the Carter's suit, too, which I dislike since I'm trying to add a layer of warmth in winter.