The typical symptom of placenta praevia is painless, unprovoked vaginal bleeding that occurs late in pregnancy or after the onset of labor. When the cervical opening is dilated in late pregnancy, the anterior part of the placenta attached to the lower part of the uterus as well as the inner cervical opening has poor ability to stretch, and it becomes misaligned and separated from the attachment site, and the blood sinus ruptures and bleeds. The amount and time of bleeding from the placenta praevia is related to the degree of placental coverage of the endocervical os. If the area covered is larger, the bleeding may occur earlier and the bleeding volume is higher. Conversely, the bleeding may be late, or even low lying placenta or borderline placenta praevia, where vaginal bleeding occurs only after labor and the amount of bleeding is low. The bleeding from anterior placenta has no obvious cause and the initial bleeding is relatively small, but the first occurrence of heavy bleeding, leading to the appearance of shock, cannot be excluded. For the very aggressive special type of placenta praevia, aggressive placenta praevia with implantation, vaginal bleeding does not necessarily occur before delivery because the placental tissue penetrates the myometrium and even reaches the bladder and the adhesions are more dense, but if the diagnosis is not clear before surgery, it may cause more critical bleeding during the operation.