Endometriosis is a disease involving the presence of endometrium outside the uterine cavity. Ectopic foci, due to their similarity to the glandular tissue of the uterine cavity, are also subject to cyclic bleeding which results in blood pooling at the site of implantation. The foci may occur in the peritoneum, ovaries or fallopian tubes, but also in the bowel, bladder or ureter.