The placenta is an important place for the exchange of materials between the mother and the fetus, and is usually round or oval, disc-shaped, bright blue, about 16-20 cm in diameter, and usually about 2.5 cm thick. The placenta is an important organ for the fetus to obtain oxygen and nutrients from the mother. One side of the placenta is the maternal side and the other side is the fetal side. The maternal side is dark red and rough. The fetal side is grayish-white, smooth and rich in blood vessels. From the general view of the placenta is round or oval in shape, with a diameter of about 16-20cm and a thickness of about 2.5cm. The placenta not only can carry out material exchange, but also has an important defense function, some pathogens or hormones and drugs in the mother’s body can’t enter into the fetus because of the placental barrier, which can protect the fetus to a certain extent.