[en] Implantation of the embryo into the maternal endometrium represents a unique biological process, combining an immunological (tolerance of an allograft) and biological (adhesion of two epitheliums) paradox. The success of implantation depends on a receptive endometrium, a functionally normal blastocyst and a synchronized cross-talk between embryonic and maternal tissues. Though sexual steroids control the process, a cascade of growth factors or cytokines are the prime paracrine mediators of the dialogue at the maternal-embryonic interface. HCG is one of the molecules most precociously produced by the embryo and is the most specific marker of its presence. HCG is a luteotropic factor which relays the inadequate support provided by the reduced rates of LH, but also influences the pregnancy on a paracrine mode by a local action on implantation process, probably by interacting with its receptor, the LH/hCG-R that we have evidenced on endometrial epithelium. We demonstrate that embryo actively participate into its implantation, tolerance and placentation.