Embryo transfer is a procedure in which a very young embryo (6-8 days old) is flushed from the pregnant mare's uterus and transferred into the uterus of another mare, to be carried to term by this second mare. The procedure is most often used in the following instances: A valuable mare which has a history of not being able to carry a pregnancy to term; a valuable performance mare at the peak of her career, when taking time out for her to have a foal would result in loss of income or loss of possibility of her excelling in her field of performance; a valuable mare of desirable breeding which would be able to produce many more foals with embryo transfer than she would if she were to carry each pregnancy to term. It must be noted, however, that not all breed registries allow the registration of more than one offspring in any given year. Still, in performance breeds, where performance of the sire and dam (not registration of the foals) drives the price commanded by the offspring, there may be sufficient incentive (in more sales) for the mare owner to consider embryo transfer.

So how exactly is embryo transfer performed? It is a fairly complex procedure, in which timing is important. It is most often a non-surgical procedure, so there is little risk of injury to the mare when an experienced veterinarian is in charge, but there must be a recipient mare available that is known to have carried foals to term. Since the recipient mare must be synchronized with the donor mare's cycle, it is helpful if the breeding facility where the procedure is to be done has a large herd of recipient mares that have proven to be able to carry a pregnancy to term. Then the donor mare must be tracked using ultrasound to determine when she will ovulate, and a recipient mare selected that will ovulate (also determined by ultrasound) 1 to 3 days before the donor mare, so that the embryo can be flushed from the donor mare (7-9 days after insemination) and implanted into the recipient mare's uterus at the appropriate time. If there are not many potential recipient mares, it is possible to coordinate the cycle of the available recipient mare with hormone therapy to match that of the donor mare, but this is more complicated than if there are many potential recipient mares to choose from.

Once the transfer to a proven recipient mare is accomplished, the pregnancy most often is uneventful. There is no greater risk of embryo loss or birth defects in embryo transfer foals than in foals carried to term by their biological mother, nor is the number of foals carried to full term different.

The costs of the procedure are not insignificant, and will vary somewhat among practitioners. It is important to remember that not all breedings result in pregnancies, so to avoid losing time and increasing costs (for veterinary services and semen collection and shipping costs from each unsuccessful breeding) one should choose a highly fertile stallion. One should also work with a veterinarian experienced in the embryo transfer procedure, with a live foal rate of near 70%, and, if possible, one that maintains a large herd of recipient mares. (Some specialists in embryo transfer offer live foal guarantees.) Costs include the procedure itself and the cost of the rent and care of the recipient mare until the embryo transfer foal is weaned.