The present invention relates to a prosthesis comprising at least two pieces of knitted fabric and to the method of manufacturing it using a sonotrode to perform ultrasonic welding. The prosthesis comprises at least one first piece of knitted fabric and at least one second piece of knitted fabric, which are welded together by means of a plurality of welds situated in a joining region in which at least a portion of the said first and second pieces of knitted fabric are superposed, the said first and second pieces of knitted fabric each comprising a group of yarns extending respectively in first and second directions A and B, characterized in that the said directions A and B are substantially aligned with one another in the said joining region, and each said weld in the joining region is of elongate shape with its longitudinal axis being aligned with the said directions A and B.