Human chorionic gonadotropin (hCG) is a glycoprotein hormone secreted by the trophoblast cells of the placenta. Its main function is to stimulate the corpus luteum, which is conducive to the continuous secretion of estrogen and progesterone to promote the formation of uterine decidua and the growth and maturation of the placenta. 

Once the embryo implants, the trophoblast cells will begin to secrete hCG, so pregnancy can be judged by the hCG value in the blood or urine.