Endometriosis is a gynaecological condition where cells similar to those that line the uterus (the endometrium) grow outside the uterus — often on the ovaries, fallopian tubes, and pelvic ligaments. These cells behave like normal endometrial tissue, thickening and breaking down during each menstrual cycle. However, because this tissue has no way to exit the body, it can cause inflammation, scarring, and pain, especially during menstruation.