Sudden blood clots underneath 18 weeks of pregnancy are likely to be caused by the occurrence of preterm miscarriage, or it may be caused by the lower position of the placenta. If you are 18 weeks pregnant and suddenly have a blood clot below, you need to pay attention to whether there is abdominal distension, lumbago and other uncomfortable symptoms. 1. If a pregnant woman feels discomfort such as abdominal cramps, lumbago, and a small amount of vaginal bleeding, it is likely to have a preeclampsia, and it is recommended to consult a doctor in time for fertility preservation treatment, so as to avoid the increase of vaginal bleeding and the progression of difficult to avoid the miscarriage of the baby. 2. If there is only vaginal bleeding and the pregnant woman does not have any discomfort, consider whether the placenta previa is present. If the placenta is in a low position, such as attached to the lower part of the uterus, the lower edge reaches or covers the cervical opening, before 28 weeks is called placenta previa, which is a common cause of painless vaginal bleeding during pregnancy. Sudden vaginal bleeding at 18 weeks of pregnancy is abnormal and prompt medical attention is recommended.