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Service Information

  • Embryo Transfer


    Embryo transfer (ET) is the removal of an embryo from the uterus of one mare (the donor) and placing it into the uterus of another (the recipient).  The recipient carries the embryo until full term,delivers the foal and nurses it until weaning.  The foal will be the genetic offspring of the donor mare despite being carried by the recipient.

    Dayrell's Miss Honey back to competing shortly after we retrieved an embryo from her last season.

    Towcester Equine Vets feel passionately about raising the standard of the British Bred Sport Horse.  Embryo transfer can help to achieve this by allowing breeders to breed from genetically superior mares without interrupting their competition schedule.  Many mares will not finish their competitive career until they are in their late teens, by which time their fertility will be declining.  Along with allowing her competitive career to continue, ET also removes the risks associated with pregnancy and parturition.

    Embryo transfer enables the mare to produce multiple offspring in any one breeding season – another way to improve the numbers of quality British bred horses.  Another benefit of the procedure is that it can overcome some uterine based infertility, enabling some mares who struggle to carry a pregnancy to full term to still pass on their genetics.

    Embryo transfer in the mare was first reported in the 1970s and has become increasingly popular in the UK in the last 5-10 years.  It is now accepted by the main registries and studbooks, with the exception of the Weatherby’s register of Thoroughbred horses.

    Recipient mare with her Embryo Transfer foal

    The recipient mare is an important part of the ET process; she must have a clean reproductive history and her uterus needs to be healthy.  The best age for a recipient mare is between 4 and 12 years old and she must be well handled with no behavioural vices.  Although there are no shared genetics between the recipient and the foal she will have an influence over the foal’s initial learned behaviour.

    Recipients should ideally be of a similar size to the donor mare.  Historically it was believed that using mares who were much larger than the donor would be of benefit but there is increasing evidence to show that this can be detrimental to postnatal growth and development.  Transferring an embryo into a mare smaller than the genetic mother can lead to a smaller foal being born, but the foal should eventually reach it’s correct size at a later stage.

    Towcester Equine Vets can offer group or individual turnout for mares visiting our stud.  We also have stabling and a horse walker should you require it.  Our stud services can also be performed at your own yard if you prefer.

    Although pregnancy rates from embryo transfer have improved significantly in recent years, they are understandably still lower than the conception rates seen with routine pregnancies.


    Successful ET requires careful synchronisation of the reproductive cycles of the donor and recipient mares. This relies heavily upon accurate monitoring of the stage of the reproductive cycle. This is achieved through ultrasonographical examinations of the reproductive tract and treatment with exogenous hormones where necessary. We synchronise at least 2 recipient mares for each potential embryo, this makes synchronisation more achievable and provides a second uterus should we get 2 embryos in the event of a double ovulation. Correct synchronisation ensures that the uterus receiving the embryo will be a couple of days behind that from which it has been removed.


    The insemination of your donor mare is ideally done at Towcester Equine Vets.  This allows us to accurately determine the day of ovulation and therefore manipulate the cycles of the recipient mares appropriately.  For the greatest success it is best to use fresh or chilled semen from a stallion of known high fertility.  It is possible to use frozen semen but this reduces conception rates and therefore is associated with a lower embryo recovery rate.


    The embryo transfer process involves two parts – flushing and transferring.  Flushing describes the removal of of the embryo from the uterus of the donor mare; a non-surgical procedure which is minimally invasive, performed 8 days after ovulation.  Once an embryo has been retrieved the transferring takes place and the embryo is placed inside the uterus of the most suitable recipient.

    An embryo being flushed from a donor mare


    The donor mare can return home and back to work immediately after the flushing has taken place. The recipient mare should stay with us for monitoring and will be scanned between 7 and 9 days later to ascertain whether she has managed to retain the pregnancy. If she is pregnant she can either go home or stay with us until her 28 day scan; at this stage we will be able to check that the pregnancy is healthy and developing normally. We recommend a further scan at 45 days. Some recipient mares may require a daily dose of Regumate throughout the earlier stages of the pregnancy.

    Claire is always happy to have an embryo safely flushed, retrieved and transferred into its recipient mare!
    A retrieved embryo

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