Progesterone is a hormone produced primarily by the corpus luteum of the ovary but also by the placenta. The action of the hormone is to cause endometrial development preparatory for the reception and nutrition of the embryo. It suppresses oullation, estrus and the secretion of LH by pituitary. During pregnancy, progesterone production by corpus luteum continues through, till near term. If pregnancy has not occurred, the output of both estrogen and progesterone suddenly fall on or about the twenty eight day of the menstrual cycle, menstrual flow starts and the uterine endometrium starts sloughing.