The implantation of embryo into the uterine walls initiates a series of pathways that suppress the maternal immune system and modulate the local immune reactions that take place in the uterus. Certain tissues present at the maternal-fetal interface are responsible for the systemic as well as local immunosuppression during pregnancy. These include:
i) Placenta: It is the tissue that connects embryo and uterine wall. It is selectively permeable, allows nutrients and other vital factors to enter the amniotic cavity, and facilitates exit of metabolic wastes.
ii) Chorion: It forms the outermost membrane that surrounds the embryo and fetus. It comprises an outer layer of trophoblast cells and an inner mesoderm layer, that is in direct contact with the amniotic fluid.
iii) Decidua: During pregnancy, the maternal uterine lining is termed as decidua. It forms the maternal component of the placenta.
Read more at Buzzle: http://www.buzzle.com/articles/why-doesnt-a-mothers-body-reject-her-fetus.html