Dydrogesterone is a synthetic progestational hormone with no androgenic or estrogenic properties. Unlike many other progestational compounds, dydrogesterone produces no increase in temperature and does not inhibit ovulation. Dydrogesterone belongs to the "retrosteroid" class and is a stereoisomer of natural progesterone. Its conformation is induced by exposing progesterone to ultraviolet light, a reaction first described in the 1950s by Reerink et al. who also discovered this new class of functionally active "retrosteroids". Dydrogesterone shares similar biological properties with natural progesterone and has high affinity for progesterone receptors [PRs]. In contrast to progesterone, dydrogesterone is highly bioavailable after oral intake and is therefore available on the market as an orally active progestogens. Progesterone was recognised early on to be one of the most important steroids required for the maintenance of pregnancy, and so dydrogesterone became a candidate for the treatment of threatened and habitual abortion. As early as the beginning of the 1980s, the pregnancymaintaining effects of dydrogesterone were demonstrated in ovariectomised rats, although the underlying mechanism was unknown. In contrast to other progestogens on the market, dydrogesterone has no relevant androgenic or antiandrogenic activity on the foetus and can therefore be safely administered to mothers without the risk of causing foetal genital malformations.