The placenta usually starts to form slowly at about 10 weeks of pregnancy. Its structure is not yet obvious and the placenta is not well developed. The smaller shape of the placenta can be faintly seen by ultrasound, and the placenta may not even be visible, usually the placenta will be fully formed after 12 weeks of pregnancy. The placenta is composed of the fetal part of the amnion and lobulated chorion and the maternal part of the fundus mater. Around the 15th-17th day after fertilization, the embryonic vessels grow into the interstitial center, intrachorionic vessels form, and the primary lobulated chorionic membrane begins to grow, gradually forming the placenta. The placenta is an important organ between the fetus and the mother to maintain the growth and development of the fetus. It has the functions of gas exchange, nutrient supply, excretion of fetal metabolites, defense against bacteria and some parasites, synthesis of trophoblast and immunity. In clinical practice, the placenta is often graded to indicate the maturity of the placenta. From late pregnancy (28 weeks), placenta grading will appear on the ultrasound report, and most of the placenta grades at this time are I-II, by about 36 weeks the placenta grade can be II, and by about 40 weeks the placenta grade can be II-III. Generally speaking, placenta grade II or above indicates fetal maturity, and if the placenta reaches grade III, it indicates that the placenta is mature and tends to age.