It's a good example of how every baby is different size-wise. He wore this Med/Long cover from the time he was about 9 months old, and still when he was 2! After he was out of diapers, I used it over his underwear to protect his pants from those little "piddles" that reminded him it was time to go. It still fits TODAY at age 3 over his underwear - and I put it on him to help take up room in his pants to keep them from falling down! Now, most babies do not fit this size range very long....he's always been skinny, and doesn't have much rump to cover!