Under normal circumstances, the endometrium is located inside the uterus, but when the endometrial tissue appears outside the uterus for various reasons, it is called endometriosis. The causes of endometriosis are not fully understood, but there are three main theories: 1. Ectopic implantation theory: women’s menstrual blood contains endometrial tissue, and when menstrual reflux occurs, the endometrium can be carried to other parts of the body, such as the ovaries and pelvis; congenital vaginal atresia and cervical stenosis can obstruct the discharge of menstrual blood, thus easily leading to menstrual reflux; cesarean section, abortion, and The endometrium can be brought to other parts of the uterus for direct implantation by multiple surgical operations such as cesarean section, abortion and tubal lavage. In addition, the endometrium can also spread to distant sites through the lymphatic and venous systems, resulting in ectopic implantation. 2. Somatic epithelial metaplasia theory: The ovarian surface epithelium and pelvic peritoneum can be activated and transformed into endometrium-like tissue when continuously stimulated by sex hormones, menstrual blood or chronic inflammation. 3. Induction theory: Undifferentiated peritoneal tissues can form endometrium-like tissues under certain endogenous biochemical induction. In addition, genetic factors, immune and inflammatory factors may also be related to the occurrence of endometriosis.