Endometriosis. Endoscopic image the pelvic wall of a woman affected by endometriosis. Endometriosis is a condition where cells from the inner lining (endometrium) of the uterus appear and flourish outside the uterine cavity, most commonly on the membrane which lines the abdominal cavity, the peritoneum. Here, it is affecting the pelvic wall. Endometriosis can cause pain and lead to infertility.