As we have already seen, the placental barrier changes during the pregnancy.
The maternal proteins do not traverse the placental barrier, with the exception of immunoglobulin (IgG), which passes over from the mother to the fetus. Through pinocytosis of syncitiothrophoblast cells the mother thus transfers to the fetus the variety of IgG that she has synthesized during her life. This transfer occurs mainly towards the end of pregnancy. Thereby the fetus obtains a passive immunity that protects it against various infectious diseases in the first six months of its life. The other immunglobulins, mainly IgM proteins, do not pass through the placental barrier.
Transferrin is another important maternal protein that, as the name indicates, transports iron. On the surface of the placenta specific receptors exist for this protein, which, by means of active transport, enters into fetal tissue.
Protein can also be transferred from the fetus to the mother; alpha-fetoprotein (the concentration of which is elevated in several fetal abnormalities) can be detected in the maternal circulation system.
Maternal or placental polypeptide hormones do not enter the fetal circulation system.