My son is 13 months old and this costume just fit.  It was snug at the legs/midsection.  I think this costume runs small overall, because my son is only 20lbs 33 inches tall.  I ordered the 2t which just fit as well.  I retured the 12-18th monthes costume and ordered a 3t-4t.  overall order a couple sizes and return the ones that don't fit.  Amazon makes returns easy.  If the 3t is too long then I will keep the 2t for my 13 month old.

Overall it is a cute costume.  the 12-18 month zips up the back, the toddler sizes just velco and arnt as thick.  Also the infant sizes the tigger head is attached (a hood) but for the toddler sizes it is a seperate piece.  Hope this helps