If there is pain in the left lower abdomen before menstruation, it is most likely due to inflammation in the adnexal area, because the pelvic cavity will be in a state of congestion before menstruation, and then the original inflammation of the adnexa will cause the painful sensation. In addition, if there is endometriosis in the left adnexal area or ovary, it may also cause local pain. Also, if there is fluid in the fallopian tube, it can cause pain in the left side of the abdomen. Therefore, ultrasound examination is recommended to find out whether there is a mass in the adnexal region or whether there is fluid in the fallopian tube, and to provide clinical treatment according to the results. In addition, if there is pain in the left lower abdomen, the possibility of ureteral calculus should be excluded.