正常情况下胎盘附着于子宫体部的后壁、前壁或侧壁。如果妊娠28周后,胎盘附着于子宫下段,甚至胎盘下缘达到或覆盖宫颈内口,其位置低于胎先露部,称为前置胎盘,而在妊娠中期B超发现胎盘前置者,称为胎盘前置状态。前置胎盘是妊娠晚期严重的并发症,也是妊娠晚期阴道流血的主要原因,据报道国外发病率是0.3%～0.9%,国内发病率为 The placenta is normally attached to the posterior, anterior, or lateral walls of the uterine body. If the pregnancy after 28 weeks, the placenta attached to the lower uterine segment, or even reach the lower edge of the placenta to cover the mouth of the cervix, its location lower than the first part of the fetus, known as placenta previa, and in the second trimester of pregnancy B-placenta previa, Known as placenta previa. Placenta previa is a serious complication of late pregnancy, vaginal bleeding is also the main reason for late pregnancy, foreign reported incidence was 0.3% to 0.9%, the domestic incidence of