Progestogens (also sometimes spelled progestagens or gestagens) are a class of steroid hormones that bind to and activate the progesterone receptor (PR). The most important progestogen in the body is progesterone (P4). Other endogenous progestogens include 17a-hydroxyprogesterone, 20a-dihydroprogesterone, 5a-dihydroprogesterone, 11-deoxycorticosterone, and 5a-dihydrodeoxycorticosterone. Synthetic progestogens are generally referred to as progestins. However, the terms progesterone, progestogen, and progestin are frequently used interchangeably both in the scientific literature and in clinical settings.