In early pregnancy, human chorionic gonadotrophin (HCG) stimulates the corpus luteum (CL) of pregnancy to produce progesterone, which in turn maintains human embryo implantation in the uterus. In addition to this embryo–maternal cross-talk via the endocrine systems through blood circulation, accumulating evidence suggests that circulating blood cells also play an important role in embryo implantation. Peripheral blood mononuclear cells (PBMC) derived from pregnant women increased the progesterone production by luteal cells and promoted the invasion of embryos in vitro. Recombinant-HCG increased chemokine production by PBMC through lectin–glycan interaction and enhanced the effects of PBMC on embryo invasion. Later, it was shown that not only PBMC, but also circulating platelets were possible sources of these chemokines that promote extravillous trophoblast invasion to reconstruct maternal endometrial artery. Circulating platelets were also proposed to induce neovascularization during CL formation. Furthermore, intrauterine administration of autologous PBMC effectively improved live birth, pregnancy and implantation rates in patients with repeated (four or more) implantation failures during in vitro fertilization therapy. These findings suggest that circulating blood cells positively contribute to maternal tissue remodeling and embryo–maternal cross-talk around the implantation period in cooperation with the endocrine system.