Pregnancy signifies, in the final sense, quite an extraordinary situation from an immunohematological perspective. It implies the existence of two immune systems in one body: the fetal and maternal immune systems. It is from this point of view, that we propose some key questions in order to allow us to truly understand how balance between both immune systems is reached. In the first place, taking into consideration that although fetal immune system is immature, it does acquire the function to develop an immune response as time goes by, so why does the fetal immune system not reject its own mother, causing a graft versus host disease (GVHD)? On the other hand, what is the immune role of placenta in this particular context? Finally, why does the mother not recognise paternal antigens in the fetus as strange and therefore reject the fetus (which could be considered an incompatible hemitransplantation or semigraft)? These questions are meant to trigger our reflexion and to allow us to bring forward several hypotheses, although in all likelihood we will not be able to reach the ultimate definite answer to these vital matters. Additionally, we will discuss implications in transfusion medicine.