I bought this for my 6 month old son who's in the 50th percentile for weight, he wears a 9-12 month size in all his clothing so I bought him the 6-12 month size in this costume....the body portion is WAY too big, but the head piece is WAY too small. My son's head is in the 40th percentile, and I can't even get the chin straps all the way around. I'm not sure if I just got a defect, but just out of curiosity I put the head piece on his twin sister (who's head is much smaller), and it was really tight on her. Too bad because it's really cute, material seems soft enough etc... but it just doesn't fit right. I don't believe this would fit on ANY 12 month old if I can't even get it on my 6 month old.