20-alpha-Dihydroprogesterone: A biologically active 20-alpha-reduced metabolite of PROGESTERONE. It is converted from progesterone to 20-alpha-hydroxypregn-4-en-3-one by the 20-ALPHA-HYDROXYSTEROID DEHYDROGENASE in the CORPUS LUTEUM and the PLACENTA.5-alpha-Dihydroprogesterone: A biologically active 5-alpha-reduced metabolite of plasma PROGESTERONE. It is the immediate precursor of 5-alpha-pregnan-3-alpha-ol-20-one (ALLOPREGNANOLONE), a neuroactive steroid that binds with GABA(A) RECEPTOR.20-alpha-Hydroxysteroid Dehydrogenase: An enzymes that catalyzes the reversible reduction-oxidation reaction of 20-alpha-hydroxysteroids, such as from PROGESTERONE to 20-ALPHA-DIHYDROPROGESTERONE.Pregnanediones: Pregnane derivatives in which two side-chain methyl groups or two methylene groups in the ring skeleton (or a combination thereof) have been oxidized to keto groups.20-Hydroxysteroid Dehydrogenases: A group of enzymes that catalyze the reversible reduction-oxidation reaction of 20-hydroxysteroids, such as from a 20-ketosteroid to a 20-alpha-hydroxysteroid (EC 126.96.36.199) or to a 20-beta-hydroxysteroid (EC 188.8.131.52).Hydroxyprogesterones: Metabolites or derivatives of PROGESTERONE with hydroxyl group substitution at various sites.Pregnanolone: A pregnane found in the urine of pregnant women and sows. It has anesthetic, hypnotic, and sedative properties.Progesterone: The major progestational steroid that is secreted primarily by the CORPUS LUTEUM and the PLACENTA. Progesterone acts on the UTERUS, the MAMMARY GLANDS and the BRAIN. It is required in EMBRYO IMPLANTATION; PREGNANCY maintenance, and the development of mammary tissue for MILK production. Progesterone, converted from PREGNENOLONE, also serves as an intermediate in the biosynthesis of GONADAL STEROID HORMONES and adrenal CORTICOSTEROIDS.Progestins: Compounds that interact with PROGESTERONE RECEPTORS in target tissues to bring about the effects similar to those of PROGESTERONE. Primary actions of progestins, including natural and synthetic steroids, are on the UTERUS and the MAMMARY GLAND in preparation for and in maintenance of PREGNANCY.Pseudopregnancy: An acyclic state that resembles PREGNANCY in that there is no ovarian cycle, ESTROUS CYCLE, or MENSTRUAL CYCLE. Unlike pregnancy, there is no EMBRYO IMPLANTATION. Pseudopregnancy can be experimentally induced to form DECIDUOMA in the UTERUS.Corpus Luteum: The yellow body derived from the ruptured OVARIAN FOLLICLE after OVULATION. The process of corpus luteum formation, LUTEINIZATION, is regulated by LUTEINIZING HORMONE.Estradiol: The 17-beta-isomer of estradiol, an aromatized C18 steroid with hydroxyl group at 3-beta- and 17-beta-position. Estradiol-17-beta is the most potent form of mammalian estrogenic steroids.Pregnancy: The status during which female mammals carry their developing young (EMBRYOS or FETUSES) in utero before birth, beginning from FERTILIZATION to BIRTH.