Normally, the placenta is attached to the posterior wall, anterior wall or lateral wall of the uterine body. If the placenta is attached to the lower segment of the uterus, or even the lower edge of the placenta reaches or covers the internal cervical os, and its position is lower than the fetal presenting part, it is called placenta previa. Placenta previa is one of the main causes of bleeding in late pregnancy and a serious complication during pregnancy. Improper treatment can endanger the lives of mother and child. Its incidence rate is reported to be 0.24% to 1.57% in China and 1.0% in foreign countries. 85% to 90% of patients with placenta previa are multiparous women, especially multiparous women, and the incidence rate can be as high as 5%.