The causes of bleeding during sex in the third trimester of pregnancy include preeclampsia, cervical polyps and so on. 1. Pre-eclampsia: contractions can be induced after sex, causing symptoms of irregular vaginal bleeding, which can also be accompanied by paroxysmal lower abdominal pain, ultrasound examination of the normal development of the fetus, combined with the history, clinical manifestations, and progesterone, blood HCG examination can assist in the diagnosis. 2. Cervical polyps: a small amount of contact bleeding can occur after sexual intercourse, not accompanied by abdominal pain, can be manifested as recurrent vaginal bleeding, gynecological examination can assist in the diagnosis of recurrent bleeding can be followed by surgical removal, and sent to the pathology examination. Bleeding during sex in the third trimester of pregnancy requires prompt hospitalization to identify the cause and actively deal with it.