I purchase the large swimmus -lrb- 22-30lbs -rrb- because my 16 month old just reach the 22lbs pound range . I return it to purchase the same in medium -lrb- 15-22lb -rrb- because my son be build very thin and have no tush . the medium fit well and I think it will still accommodate he for awhile . even if it do not and I have to re-purchase the large , it be still cheaper than have to buy a pack of disposable swim diaper .