Brown blood clots in the vagina of a 33-week pregnant woman may be due to preterm labor, placenta previa, placenta praevia and gynecological inflammation and other possibilities. 1. Preterm labor: preterm labor can be manifested as vaginal bleeding, abdominal pain and other symptoms, need to consult the doctor in time, the appropriate treatment to protect the fetus. 2. Placental abruption: it is one of the complications after 20 weeks of pregnancy or during labor, which may be related to vascular lesions of pregnant women, abdominal trauma and other factors. It is mainly manifested by vaginal bleeding, mostly non-clotting blood, accompanied by abdominal pain, fetal heart rate changes and other symptoms. 3. Placenta praevia: It is one of the late pregnancy complications after 28 weeks of pregnancy, the etiology of which is still unclear and may be related to endometrial damage, placental abnormalities and other factors. Mainly can appear painless vaginal bleeding, the phenomenon of brown blood clots. 4. Gynecological inflammation: If pregnancy is combined with vaginitis, cervicitis, cervical erosion, cervical polyps and other diseases, vaginal bleeding during pregnancy can also occur. 33 weeks pregnant women vaginal brown blood clots is an abnormal phenomenon, it is recommended to go to the hospital in a timely manner to identify the cause of the disease to take symptomatic treatment.