So we have two structures that are capable of releasing progesterone. Initially it's the corpus luteum, but eventually when the corpus luteum degenerates during the process of pregnancy, it's the placentha that releases our progesterone. Now, what exactly is the function of progesterone? So it has many important functions and one of the most important functions is the maintenance of the endometrium. So it's the estrogen that generates and thickens that endometrium, but it's progesterone that maintains the endometrium as our ovum travels through the fallopian tube and during the process of pregnancy. So progesterone is used to maintain the endometrium during the menstrual cycle as well as during pregnancy if the sperm actually combines with our egg.