Pregnant women experience many physiological changes before implantation of the early embryo (blastocyst) takes place. Ovulation, copulation, and fertilization directly or indirectly induce dramatic changes in uterine physiology that resemble classical inflammation at the mucosal surfaces of the female reproductive tract, and it is quite likely that these changes impact the maternal immune system well before the blastocyst implants in the uterus. Consequently, the outcome of the immune response differs during pregnancy, when compared to outcomes in nonpregnant women. Thus, the uterus may be preconditioned to accept the blastocyst.

Blastocyst implantation is a crucial point in the process of reproduction because it is the moment of highest spontaneous embryo loss for humans. It is characterized by the invasion of trophoblastic cells in the maternal decidua, a mucosal tissue derived from the endometrium. Antigenically, the fetus and placenta have half of the histocompatibility genes because of the...