Progesterone is manufactured primarily by the corpus luteum which is the follicle that develops after ovulation. A small amount of progesterone is also produced to a small degree by the adrenals. In the ovary, progesterone production is activated at ovulation 12-14 days before the next menstrual cycle, stimulated by the release of luteinizing hormone from the pituitary gland and is crucial to the survival of the ovum once fertilized. When pregnancy occurs, progesterone production increases rapidly to support the embryo. Once the pregnancy is established, Progesterone’s manufacture and production is taken over by the placenta. If a woman does not get pregnant, the corpus luteum involutes and progesterone production diminishes and eventually disappears in parallel with estrogen production which prompts the menstrual cycle to start. Once this occurs, the monthly process repeats itself.
Progesterone is a precursor to most sex hormones, including estrogen in the ovaries, testosterone, all androgens, and other adrenal hormones, making it an extremely important hormone for reasons far beyond its role as a sex hormone.
Progesterone in the breast and uterus counteracts the stimulation of cell growth and proliferation, which is a direct action of estrogen. It accomplishes this action by activating the progesterone receptor, which in turn, down-regulates the estrogen receptor. Because progesterone suppresses estrogen-driven cell growth and proliferation, progesterone in the natural state helps keep breast and uterine cell growth in healthy balance decreasing the risk of breast and uterine cancer .