The equine uterine environment plays an integral role in the early developmental stages of pregnancy, even prior to implantation of the embryo, according to British researchers who recently examined the role of the uterus in embryo transfer success.

According to the study, "Uterine influences on embryogenesis and early placentation in the horse revealed by transfer of day-10 embryos to day-3 recipient mares," the ability of an equine conceptus (early embryo) to overcome a seven day asynchrony with the uterus is unique to horses. This seven day asynchrony was created by non-surgically transferring 10-day-old embryos from pregnant mares to recipient mares that ovulated seven days after the donor mares.

The transferred conceptuses grew slower compared to control mares with normal pregnancies. In addition, the development of the endometrial cups (structures that grow from the fetus into the lining of the mare’s uterus and produce the hormone equine chorionic gonadotropin, which is thought to play a role in pregnancy maintenance) occurred approximately seven days later than in the control mares. But the conceptuses remained viable.

Based on these results, the authors concluded that the equine conceptus is unique among mammals in that it is capable of overcoming a seven day asynchrony of the uterus (between donor and recipient mares). How this translates into a clinical application remains to be determined, but it could mean that mares do not need to be perfectly synchronized to achieve a successful embryo transfer