If its just size you are not happy with you will have to pay. MYA will only re operate for free if there is a medical problem or complication like capsular contracture. If you do want a bigger size and are happy to pay again most surgeons will make you wait a year before a re-op as they have to wait for the body to completely heal from the first op before they go in again as it has too many risks associated with it. At 13 weeks you are still in the early phases of recovery though and you may find that they continue to change especially during the first 6 months post op.
