An implantable prosthesis and a system and method for repairing, or resisting the formation of, a hernia at an opening or stoma through an anatomical structure such as the abdominal wall. The implantable prosthesis includes a body portion and an opening therethrough, that is adapted to receive an element, such as a section of bowel, that is being externalized through the anatomical structure. The system includes the implantable prosthesis and a cannula having an outer dimension sized to fit within the opening in said implantable prosthesis, and an inner dimension sized to pass through the element. The implantable prosthesis may include a reinforcement member, and it may also include flaps that extend into the opening of the implantable prosthesis. The method includes inserting the cannula into and beyond the opening or stoma, and locating the opening of the prosthesis about the end of the cannula. A portion of the element to be externalized, such as a section of bowel in an enterostomy, is drawn through the end of the cannula such that the bowel section does not contact the implantable prosthesis. The cannula is then removed.