Early pregnancy in the horse revisited – does exception prove the rule?

Early equine pregnancy shares many features with that of more intensively assessed domestic animals species, but there are also characteristic differences. Some of those are poorly understood. Descent of the equine conceptus into the uterine lumen occurs at day 5 to 6 after ovulation but is only possible when the embryo secretes prostaglandin E2. Although maintenance of equine pregnancy probably involves secretion of a conceptus derived anti-luteolytic factor, this agent has not been identified. Rapid growth, conceptus mobility and presence of an acellular capsule at the time of maternal recognition of pregnancy, i.e. between days 12 and 14, are prerequisites to avoid pregnancy loss. Progesterone together with 5α-pregnanes is secreted by the corpus luteum and induces the production of endometrial histotroph which is responsible for conceptus nutrition until placention. A stable contact between the outer trophoblast layer of the allantochorion and the luminal epithelium of the endometrium is not established before days 40 to 42 of pregnancy.