Bought these for my 4 1/2 month old son who weighs 16lbs and is 26inches long. Purchased 9-12 months and they fit on the waist because of the elastic but were big all the way through and seemed to fit just right in length so Id say they definitely would be short and look silly on him if he were actually 9-12 months. I hate the straight leg look (save yourself the trouble and get tapered leg). These are even worse after washing they look like bootleg sweats and he doesnt wear them in public because they look funny. Not stylish to my taste and of poor quality.