Diagnostic criteria of placenta praevia: According to the relationship between the lower edge of the placenta and the endocervix, there are 4 types of placenta praevia: 1. complete placenta praevia: the endocervix is completely covered by the placenta, and the placenta is echoed above the endocervix on transverse scan, and there is no amniotic fluid gap; 2. low placenta: the placenta occupies the lower 1/3 of the uterus, the edge of the placenta has not reached the endocervix, and the distance between the lower edge of the placenta and the endocervix is < 2 cm; 3. Marginal placenta praevia: the placenta is located in the lower 1/3 of the uterus, the lower edge of the placenta is flat and does not cover the endometrium; 4. Partial placenta praevia: the endocervix is partially covered by the placenta.