These are all the trim fit cover that fits amazing over Eflats and Flats. The Os fit approximately 10-45lbs so will fit for a long time. Newborn/Small will fit from approximately 5-16lbs. Tummy elastic for those tummy sleepers. I have personally used this cover for my littles and they are the best fitting cover I have tried. I have used a lot of different patterns before I found one that was such an awesome design. The inner is smooth pul so fully wipeable for easy clean up.