ABSTRACT
The placenta is far from a mere filter between the mother and the fetus; rather, it is an extremely important organ which protects and feeds the embryo. It contains and produces a wide variety of biologically active compounds which are for the most part proteins: hormones, enzymes, transport and storage proteins, receptors or structural proteins, and so on. The products of the fetomaternal unit, including estriol, have been assigned great significance in the preservation of intrauterine fetal well-being. To date only in vitro examination has substantiated the protective, immunosuppressive effect of substances of fetal origin like Alpha-Fetoprotein , or of pregnancy hormone changes, primarily the maternal-origin serum factors like a2-PAG, which are elevated by estrogen and protect the fetal allograft during pregnancy. Human chorionic gonadotropin was the first pregnancy specific protein to be identified, and its hormonal effect has been the basis of numerous pregnancy tests since 1927.
