I. Principle of placenta praevia bleeding: It is due to the gradual change of the cervix and lower uterine segment and contractions, which will produce shearing force on the inelastic placental attachment site, resulting in partial abruption of the placenta, thus causing bleeding. The source of bleeding is mainly maternal blood in the intervillous space, but if the fetal blood vessels in the terminal villi are broken, there will also be fetal blood. If the source of bleeding is fetal blood then fetal distress may occur rapidly. Second, the actions that cause bleeding from placenta praevia: 1. Vaginal examination: it may damage the placenta and the uterine meconium between the placenta and cause bleeding, so. If the diagnosis of placenta praevia, vaginal examination or anal fingering is generally prohibited. 2, sexual behavior: stimulation of the cervix can damage the placenta locally causing bleeding. Third, the manifestation of placenta praevia bleeding and factors affecting it: 1, pain and bleeding: In late pregnancy, the most common symptom of placenta praevia is relatively painless vaginal bleeding, which occurs in up to 90% of persistent cases. 10%-20% of patients show contractions, pain and bleeding, similar to placental abruption. 2. Gestational week and bleeding: About 1/3 of placenta praevia have their first vaginal bleeding before 30 weeks of gestation; these patients may require blood transfusion and are at higher risk of preterm delivery and perinatal death. About 1/3 bleed at 30-36 weeks of gestation. The remaining 1/3 mostly have their first vaginal bleeding after 36 weeks of gestation. About 10% of patients do not bleed until full term. 3. Placental attachment site and bleeding: Compared to posterior or lateral wall placenta praevia, anterior wall placenta praevia is more likely to cause antepartum bleeding and greater bleeding during cesarean delivery. Any antepartum hemorrhage is a risk factor for preterm labor prodromal and premature rupture of membranes before term. 4. Ultrasound manifestations of increased risk of hemorrhage: placenta covering rather than near the endocervix; placental rim thicker than 1 cm, angle between the basal plate and the chorionic plate greater than 45 degrees, presence of an echogenic zone at the edge of the placenta covering the endocervix, cervical shortening shorter than 3 cm, and cervical shortening in late pregnancy.