The placenta before 28 weeks of pregnancy, that is, before 7 months of age, is grade 0. The placenta is between the mother and the fetus, and consists of the bottom meconium, lobulated chorionic membrane and amniotic membrane. The placenta is round or oval in full term, about 16-20cm in diameter, 1-3cm thick, thick in the middle and thin in the periphery, weighing about 450-650g. Synthesis function, barrier function is limited, placenta from occurrence, development, growth, maturation, aging process is divided into 4 levels, namely 0-3 levels, placenta before 28 weeks is level 0, placenta from 28-36 weeks is level 1, placenta from 36-40 weeks is level 2-3, so level 0 placenta is the placenta before 7 months.