Four separate components combine to produce the progesterone and biologically active 5α-reduced pregnanes needed to maintain pregnancy in the mare. The primary corpus luteum (CL) is prolonged beyond its cyclical lifespan by the down-regulation of endometrial oxytocin receptors to prevent activation of the luteolytic pathway and its waning progesterone production is supplemented from day 40 of gestation by the formation of a series of accessory CL which develop in the maternal ovaries as a result of the gonadotrophic actions of pituitary FSH and the equine chorionic gonadotrophin (eCG). From around day 100 the allantochorion secretes progesterone and progestagens directly to the endometrium and underlying myometrium and, in the last month of gestation, the enlarging foetal adrenal gland secretes appreciable quantities of pregnenelone which is also utilized by the placenta to synthesize progestagens. Between 10 and 15% of mares undergo foetal death and abortion at some time in gestation and the majority of these losses occur during the first 40 days of gestation when the primary CL is the sole source of progesterone. Yet, all the available evidence suggests that untoward luteolysis is not common in this period and the losses that do occur have other underlying causes. Beyond day 40 the secondary CL receive powerful luteotrophic support from eCG and from day 80–100 until term the supply organ (placenta) and target tissues (endometrium and myometrium) are in direct contact with each other over their entire surface. In the face of this interlocking and failsafe system for progestagen production throughout pregnancy, and despite a paucity of evidence that a deficiency of progesterone production is a cause of pregnancy loss in the mare, it is surprising, and worrying, that annually many thousands of pregnant mares throughout the world are given exogenous progestagen therapy during part or all of their gestation as a form of preventative insurance against the possibility of pregnancy failure. Basic investigative research is required urgently to validate or debunk the practice.