Hydroxyprogesterone (17-OH progesterone or 17OHP) is a C-21 steroid hormone produced in the synthesis of glucocorticoids and sex steroids. It is derived from progesterone via 17-hydroxylase, a P450c17 enzyme, or from 17-hydroxypregnenolone via 3β-hydroxysteroid dehydrogenase/Δ5-4 isomerase. As a hormone, 17OHP also interacts with the progesterone receptor. 17-Hydroxyprogesterone is not the same compound as 17-hydroxyprogesterone caproate. 17-Hydroxyprogesterone is a natural progestin and in pregnancy increases in the third trimester primarily due to fetal adrenal production. 17-Hydroxyprogesterone caproate is a synthetic (artificial) hormone that is similar in structure to medroxyprogesterone acetate and megestrol acetate.