Blood in the underpants in the fifth month of pregnancy is caused by the low position of the placenta, which needs to pay attention to appropriate rest, preeclampsia caused by the need to give medication to protect the fetus, cervical polyps caused by the need for observation or polyp removal surgery. 1. Low position of placenta: when the ultrasound examination in the fifth month of pregnancy indicates that the position of placenta is low, it can cause the symptom of blood stains in the underwear after activity, which is usually not accompanied by abdominal pain, and the combination of clinical manifestations and ultrasound examination can assist in the diagnosis. It is necessary to pay attention to appropriate rest, reduce the amount of activity, prohibit sexual intercourse, and follow the doctor’s instructions to review the ultrasound on time. 2. Pre-eclampsia: It can also cause the symptom of blood in the underwear, and may be accompanied by lower abdominal pain, which can be diagnosed by combining clinical manifestations and ultrasound examination. To follow the doctor’s advice, appropriate rest, and give magnesium sulfate or ritodrine hydrochloride and other drugs to protect the fetus treatment, pay attention to personal hygiene, keep the vulva clean and dry. 3. Cervical polyp: It can also cause small amount of vaginal bleeding, not accompanied by abdominal pain, combined with clinical manifestations and gynecological examination can assist in the diagnosis. Generally do not need to deal with, but recurring need to perform polyp removal surgery, and the removed tissue will be sent to the pathology. When there is blood in the underpants in the fifth month of pregnancy, you need to go to the hospital to find out the cause and treat it in time.