Progestogens (also spelled progestagens or gestagens) are a group of hormones including progesterone. The progestogens are one of the five major classes of steroid hormones, in addition to the estrogens, androgens, mineralocorticoids, and glucocorticoids. All progestogens are characterized by their basic 21-carbon skeleton, called a pregnane skeleton (C21). In similar manner, the estrogens possess an estrane skeleton (C18) and androgens, an andrane skeleton (C19). Progestogens are named for their function in maintaining pregnancy (pro-gestational), although they are also present at other phases of the estrous and menstrual cycles...