It is both an endogenous steroid and a sex hormone. It plays crucial roles in the menstrual cycle and in maintaining the early stages of pregnancy. It is mainly released by the corpus luteum a temporary endocrine gland in the ovaries formed from the empty ovarian follicle after ovulation. During pregnancy, it not only prepares the endometrium (the mucous membrane lining the uterus) for the potential of pregnancy after ovulation by triggering the lining of the uterus to thicken in preparation the reception of a fertilized egg, but it also stimulates the growth of maternal breast tissue. Moreover, it prevents the rejection of an egg as it inhibits muscle contractions of the pelvic wall. When the production when the production of progesterone declines, the lining of the womb breaks away and menstruation begins.
See Cholesterol, Estrogen, Fallopian tubes, Hormones, Menarche, Ontogenetic adaptation, Steroids, Testosterone