After 28 weeks of gestation, the placenta is attached to the lower part of the uterus, and the lower edge reaches or covers the inner cervical opening, below the previa of the fetus. It is the most common cause of vaginal bleeding in late pregnancy and is one of the serious complications of pregnancy. The etiology is still unknown and may be related to the following factors: 1. Abnormal placenta: abnormal placental morphology and placental size. The placenta is in a normal position, but the parietal placenta is located in the lower part of the uterus near the endocervix; the placenta is too large and the membranous placenta is large and thin extending to the lower part of the uterus; the incidence of placenta praevia in twin pregnancies is one times higher than in singleton pregnancies. 2, endometrial lesion or injury: cesarean delivery, history of uterine surgery, history of multiple abortions and scrapings, puerperal infection, pelvic inflammatory disease, etc. can cause endometritis or atrophic lesions. After pregnancy, the placenta will fully expand its area in order to obtain sufficient nutrition, so that the placenta reaches the lower segment of the uterus or the endocervix, resulting in placenta praevia. 3, delayed development of the trophoblast layer of the fertilized egg: when the fertilized egg enters the uterine cavity, the trophoblast layer has not yet developed to a stage where it can be laid, and the fertilized egg continues to move down due to gravity, and when it reaches the lower part of the uterus, its development is synchronized with the endometrium, and it lays in the lower part of the uterus and develops into the placenta praevia. 4. Assisted reproductive technology: The use of ovulation-promoting drugs changes the level of sex hormones in the body. Due to in vitro culture and artificial implantation of the fertilized egg, the endometrium is not synchronized with the development of the embryo, and contractions are induced during artificial implantation, resulting in its implantation in the lower uterine segment. Risk factors for the occurrence of placenta praevia include history of multiple abortions, history of uterine operations, history of puerperal infection, advanced age, history of cesarean section, history of multiple births, poor lifestyle habits of pregnant women (women who smoke or use drugs), twin pregnancies, conception with assisted reproductive technology, and abnormal uterine morphology.