3.2.2. Chorionic Gonadotropin

Once a competent blastocyst makes contact with the maternal endometrium, a dialogue made of signals and responses between them occurs. One of the most important factors secreted by trophoblast cells is CG. CG is expressed very early by the embryo, since its mRNA can be detected starting from the 6–8 cell stage. The protein is secreted by both zona enclosed or hatched blastocysts, and is independent of blastocyst interaction with the endometrium [242]. During pregnancy, CG is firstly detectable in maternal blood during implantation and then rapidly increases [243]. As discussed before, CG plays a fundamental role in inducing the production of progesterone and mediating the decidualization process, thus allowing implantation of the blastocyst.
