Oestrogens and pregnancy maintenance in the mare: For or against?

Abstract

It has been proposed that oestrogen secreted in large quantities by the rapidly expanding blastocyst is responsible for maternal recognition of pregnancy in the mare, as it is in the sow. However, while exogenous oestrogen administration readily prolongs luteal lifespan in dioestrous sows, the recorded effects on dioestrous mares have ranged from the predicted luteostasis to a diametrically opposed, hastened luteolysis. One possible explanation for the contrary results is that, as in the domestic ruminant species, oestrogens (of ovarian origin) actually help to establish the luteolytic cascade. If so, high systemic oestrogen concentrations might be expected to compromise rather than to protect early pregnancy, and there is some evidence that this may be the case. On the other hand, conceptus oestrogens almost certainly have important local intra-uterine effects, for example on myometrial activity, vascularity and endometrial gland secretion, all of which are essential to equine conceptus development.