As the embryo grows, the amniochorion and its associated decidua fuse with the opposite uterine wall, so that by the fourth month the uterine cavity is entirely taken up by the fetus. The chorion is also an important endocrine gland. It also produces a hormone that supports lactation. The developing fetus is a foreign object to the mother. The trophoblast of the fetus actually invades the uterus, but the mother does not have an immune response against the fetus.
Immunosuppression appears to be a function of the chorion.
It both secretes proteins that block the production of antibodies and promotes the production of types of lymphocytes that suppress the immune response within the uterus.