The pain in the stomach during menstruation is often due to the following reasons: First, the patient is sensitive to prostaglandins, which are secreted in the body during menstruation and cause pain in the lower abdomen due to uterine contraction. Secondly, inflammation of the pelvic cavity can cause pain in the lower abdomen when menstruation occurs because the pelvic cavity becomes congested with blood when menstruation occurs, which causes pain from the original inflammation. Third, menstrual pain caused by endometriosis, such as endometriosis in the ovaries or ectopic in the pelvis, or ectopic in the myometrium, the foci of ectopic disease will also bleed when menstruating, and the clinical symptoms of stomach pain will appear. Fourth, if the uterus is malformed, it will also lead to stomach pain during menstruation.