A molecular model of progesterone, a steroid hormone produced in the ovaries, adrenal glands and the placenta. Progesterone prepares the uterus for implantation as the endometrium enters the secretory stage, and will decrease if pregnancy does not occur, leading to menstruation. Progesterone levels will remain high during pregnancy to decrease uterine smooth muscle contraction and inhibit lactation until after delivery. Atoms are colored dark gray (carbon), light grey (hydrogen) and red (oxygen).