The placenta is one of the fetal appendages. Fetal appendages include the placenta, amniotic fluid, and umbilical cord. The placenta supplies nutrition, nutrients and oxygen to the fetus through the blood vessels in the placenta, and the fetus can also excrete some waste through the placenta. At the same time, the placenta has a protective effect on the fetus, it acts as a placental barrier, there are many substances that can only enter the maternal blood circulation and are shielded from reaching the fetus by the placenta, so it has a protective effect on the fetus. There are also some small molecules or substances that can cross the placental barrier and reach the fetus for therapeutic effects, such as dexamethasone, which can cross the placenta to promote fetal lung maturation.