Brown discharge in the third trimester of pregnancy may be caused by factors such as preeclampsia, placenta hypoplasia, and gynecological diseases. 1. Pre-eclampsia: In the third trimester of pregnancy is the early stage of pregnancy, in the daily care is not appropriate or the body of progesterone is insufficient, may induce pre-eclampsia, there will be a vaginal brown discharge, with or without pain in the lower abdomen, need to consult a doctor in a timely manner, if necessary, the treatment of fetal preservation. 2. Placenta hypoplasia: the low position of the placenta during pregnancy may cause vaginal bleeding, the main symptom is painless recurrent vaginal bleeding. 3. Gynecological diseases: For example, cervical polyps, vaginitis and other diseases will lead to redness, swelling, congestion and other symptoms in the affected area. And once irritated, it may ulcerate and then the symptoms of vaginal bleeding will appear. Brown discharge in the third trimester of pregnancy indicates the presence of abnormalities, and pregnant women are advised to seek medical attention as soon as possible for examination and treatment to avoid serious complications.