What hormone does the corpus luteum secrete to keep the placenta a rich, thick bed of blood vessels?

The corpus luteum forms after the release of an egg from the ovary, and it secretes progesterone which helps to develop and maintain the endometrium, or the inner lining, in the uterus.

If an egg is not fertilized, the corpus luteum stops producing progesterone after about 10 days in humans, at which time the lining of the uterus is shed. If an egg is fertilized and implants in the lining of the uterus, the developing ball of cells produces human chorionic gonadotropin (hCG) hormone which signals the corpus luteum to continue producing progesterone. In this way, the lining of the uterus is maintained and provides a healthy environment for a fetus to grow along with the placenta. Eventually the placenta will take over the role of producing progesterone, but the corpus luteum is essential at the beginning of the process to maintain a pregnancy.