The vague pain in the left lower abdomen before menstruation is often due to the following reasons: 1. The left adnexa had chronic inflammation, local congestion and edema when the adnexa has chronic inflammation. And before menstruation, the pelvic cavity will also be congested, it will aggravate the original inflammatory congestion symptoms, so there will be vague pain in the left lower abdomen. This condition also causes pain in daily life, especially after exertion or sexual intercourse, the pain tends to increase.2. It is caused by left-sided endometriosis, such as ovarian endometriosis, which can also appear as vague pain before menstruation, and patients often have a history of dysmenorrhea. Patients are advised to do ultrasound and gynecological examination after menstruation is clean for preliminary diagnosis.