The placenta maturity is classified as grade 0-3 according to the parenchymal echogenicity of the placenta, the characteristics of the chorionic plate and the presence or absence of calcification points under ultrasound. Grade 0 placenta is a placenta with even distribution of parenchyma, no calcification points, smooth and clear villi plate. grade 1 placenta: coarse grainy strong echogenicity in the placenta parenchyma, dotted calcification visible, clear villi plate with slight ripple-like undulation. grade 2 placenta: short columnar strong echogenicity in the placenta parenchyma, cut marks in the villi plate, but not reaching the basal layer, dotted linear strong echogenicity. grade 3 placenta: strong echogenic ring in the parenchyma, patchy strong echogenicity with posterior echogenicity. The placenta is characterized by strong echogenic rings within the parenchyma, patchy strong echogenicity with posterior acoustic image, tangential traces of the chorionic plate reaching the basal layer, and a large amount of strong echogenicity within the basal layer.