Dydrogesterone (INN, USAN, BAN), is also chemically known as 9β,10α-pregna-4,6-diene-3,20-dione. Dydrogesterone (6-dehydro-retroprogesterone) is a hormonally active, non-androgenic steroid that was developed in the 1950s.Dydrogesterone has selective progestational activity and does not inhibit ovulation. The greater rigidity of dydrogesterone also positively affects its selectivity, while natural progesterone is less selective, existing in different conformations that more easily bind to different receptors. As a consequence of its better bioavailability and the progestational activity of its main metabolites (20-, 21- and 16-hydroxy derivatives), the equivalent dose of dydrogesterone is 10–20 times lower than that of oral micronized progesterone.Dydrogesterone is used as an effective, orally active progestogen for gynaecological conditions related to a wide variety of progesterone deficiencies in pregnant women. The molecular structure and pharmacological effects are somewhat similar to endogenous progesterone, although in smaller amounts it is found to be orally active. Its freedom from hormonal effects like those related to corticoid, androgenic, estrogenic, anabolic, and other effects gives dydrogesterone an advantage over other synthesized progestogens.Dydrogesterone when used therapeutically is closely related to its physiological action on the neuro-endocrine control of ovarian function, as well as on the endometrium. This is an indication in all cases of endogeneous progesterone deficiency - relative or absolute. The molecule was licensed for use in several indications, including threatened or recurrent miscarriage, dysfunctional bleeding, infertility due to luteal insufficiency, dysmenorrhea, endometriosis, secondary amenorrhoea, irregular cycles, pre-menstrual syndrome and also as a hormone replacement therapy.Dydrogesterone has proven effective in the following conditions associated with progesterone deficiency: Infertility due to luteal insufficiency Threatened miscarriage Habitual or recurrent miscarriage. Menstrual disorders Premenstrual syndrome Endometriosis Dydrogesterone has also been registered as hormone replacement therapy (HRT) to counter-check the negative effects of unopposed estrogen on the endometrium in women with an intact uterus. Dydrogesterone is relatively safe and well tolerated, and does not exhibit the androgenic side effects that are common with some other progestins, like medroxyprogesterone acetate.