CD4 and Miscarriage: Research indicated that, on the one hand, NK cells promote implantation, placenta formation, and spiral artery remodeling, yet, on the other hand, overactivated NK cells can enhance the toxic effect on the embryo and cause miscarriage 12 Meanwhile, Tregs are a subset of pregnancy protective cells, belonging to a subset of special immunosuppressive CD4+T helper cells, which exert the inhibitory function in early pregnancy and can directly inhibit the killing of embryos by effector T cells, and secrete a variety of immunosuppressive factors to regulate other immune cells 52.