The placenta is only gradually formed at 10 weeks of pregnancy, its structure is not yet obvious and its function is not yet perfect, the small shape of the placenta can be vaguely seen through ultrasound, and even the placenta may not be visible, generally the placenta will be fully formed after 12 weeks of pregnancy, with the functions of material exchange, defense, synthesis, immunity, storage and metabolic regulation, etc. It can secrete a large amount of estrogen and progesterone, replace the function of ovarian corpus luteum, and maintain normal pregnancy. The placenta is composed of amniotic membrane, amniotic fluid, amniotic fluid and amniotic fluid. The placenta is mainly composed of the amniotic membrane, lobulated chorionic villi and the basal meconium, of which the amniotic membrane constitutes the fetal part of the placenta and is the innermost layer of the placenta. The amnion constitutes the fetal part of the placenta and is the innermost layer of the placenta. The basal meconium constitutes the maternal part of the placenta and accounts for a very small part of the placenta at term. The placenta is generally divided into Ⅳ grades after formation, as follows: 1.Grade 0 placenta: the placenta is homogeneous and fine-grained, common in early and middle pregnancy; 2.Grade I placenta: the placenta is homogeneous and coarse-grained, common in middle pregnancy; 3.Grade II placenta: the placenta has a tendency to form lobules, but has not yet formed placental lobules, common in middle and late pregnancy; 4.Grade III placenta: mature placenta, with placental lobule formation, sometimes with calcification It is common in late pregnancy.