My grandson is 17 months old and 25 lbs.and 32 inches.  He is just getting into the 18 month clothes and they fit fine after the first washing.  He is slender, and what we like about Gerber, is that the waist fits him.  With other brands, the waist is too big and he gets "droopy drawers".  If your child is chubby or thick-waisted, go with one size up and roll the sleeves up.  As another customer put it, "Gerber makes clothes for slender tall children".