Endometriosis is a gynecological medical condition in which cells from the lining of the uterus (endometrium) appear and flourish outside the uterine cavity, most commonly on the membrane which lines the abdominal cavity. The uterine cavity is lined with endometrial cells, which are under the influence of female hormones. Endometrial-like cells in areas outside the uterus (endometriosis) are influenced by hormonal changes and respond in a way that is similar to the cells found inside the uterus. Symptoms often worsen with the menstrual cycle. Endometriosis is typically seen during the reproductive years; it has been estimated that endometriosis occurs in roughly 610% of women. Symptoms may depend on the site of active endometriosis. Its main but not universal symptom is pelvic pain in various manifestations. Endometriosis is a common finding in women with infertility.