The reasons for the formation of complete placenta praevia are the same as those for each category of placenta praevia, mainly as follows: 1. The pregnant woman has a previous history of multiple pregnancies with scraping, a history of multiple deliveries, cesarean deliveries, or uterine fibroid peeling. These conditions cause damage to the endometrium, and the placenta cannot get sufficient blood supply to supply the fetus in the bed position, it will extend and even reach the lower part of the uterus to completely cover the endocervix and cause complete placenta praevia; 2, smoking, alcohol and drug abuse can also induce placenta praevia; 3, the risk of placenta praevia in advanced maternal age is also greater than that in ordinary maternity; 4, multiple pregnancies In order to supply enough nutrients to multiple fetuses, the placenta must be large enough to absorb enough nutrients, so the placenta will also extend to the lower part of the uterus; 5, the fertilized egg is slow to develop, and when the fertilized egg reaches the uterine cavity, it is not ready to implant, and continues to move down to the lower part of the uterus to complete implantation, so that the position of the placenta will be low and cause placenta praevia.