The syncytiotrophoblast also produces human chorionic gonadotropin (hCG), a hormone that stimulates the release of progesterone from the corpus luteum. Progesterone enriches the uterus with a thick lining of blood vessels and capillaries so that it can sustain the developing embryo. The villi begin to branch and contain blood vessels of the embryo. Arteries in the decidua are remodelled to increase the maternal blood flow into the intervillous spaces of the placenta, allowing gas exchange to take place as well as the transfer of nutrients to the embryo. Waste products from the embryo will diffuse across the placenta.