Embryo transfer (ET) is an advanced reproductive technology that allows breeders to produce multiple offspring from a single mare in a given year. This procedure involves taking an embryo from a donor mare, and depositing it in the uterus of a recipient mare who carries the pregnancy to term.

ET is becoming increasingly common in equine reproduction, and many equine veterinarians are able to perform the technique for horse breeders. However, embryo transfer requires extensive management of both the donor mare and the surrogate mare to have the greatest chance of pregnancy.

To perform embryo transfer, the donor mare is typically impregnated by artificial insemination to produce a fertilized embryo. Between Day 7 – 9 of gestation, the veterinarian flushes the embryo from the donor mare’s uterus, and prepares it for implantation into a synchronized recipient mare. The recipient mare’s ovulation must occur within 4 to 8 days of the donor mare’s ovulation.

Embryo transfer has a high success rate, with over 90% of embryo transfers successfully implanting in the recipient mare. [4] This article will discuss the process of retrieving and transferring embryos, as well as benefits and considerations of this reproductive technology.

Embryo Transfer in Horses

Embryo transfer is the process of taking an embryo from a donor mare, and transferring it into a recipient mare who will carry the pregnancy to term. It is essentially a form of surrogate pregnancy for horses.

The ability to have a different mare carry a pregnancy has many benefits, including: [1][2][3]

  • Producing foals from performance and show mares currently competing
  • Having multiple foals from the same mare in one year
  • Ability to produce foals from mares with health issues or musculoskeletal problems, which prevent them from carrying their own foals
  • Ability to produce foals from mares with reproductive health issues that prevent them from sustaining a pregnancy

However, not all breed organizations allow registration of embryo transfer foals, including the Jockey Club, which registers racing Thoroughbreds. [1] Horse breeders must investigate their breed’s specific regulations around assisted reproductive technologies before pursuing embryo transfer.

Stages of Embryo Transfer

There are three stages of embryo transfer that must be successful for pregnancy to occur. These stages are:

  • Pregnancy in the donor mare to produce the desired embryo
  • Flushing of the embryo from the donor mare
  • Implanting the embryo into a synchronized recipient mare at the same stage of estrus as the donor mare

Recent advances in embryo transfer include the ability to cool or freeze embryos for storage or transport. Embryo cooling is widely accessible, and can be performed by most equine reproductive veterinarians. Embryo freezing is more complex and may only be available from certain specialists.

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Donor Mare Management

Most donor mares are bred using artificial insemination, as this allows the veterinarian to precisely identify breeding dates and track the mare’s estrus cycle. [1]

Once fertilization occurs, the veterinarian continues to track the embryo’s development to identify the best time to perform a uterine flush, which removes the embryo from the donor mare. [1]

Embryos flushed around 7 days after ovulation are usually the ideal size for embryo transfer, however its size can vary depending on the mare’s age, stallion’s fertility, timing of breeding, and individual variation in the reproductive tract of the mare. [1]

Ultimately, the veterinarian must decide the best time to flush the embryo based on the results of ultrasounds of the reproductive tract.

Uterine Flushing

Historically, surgical techniques were the only available option for embryo recov