Introduction to Embryo Transfer



Embryo Transfer – A Summary


Equine embryo transfer has become a useful tool in the horse breeder’s arsenal.  Common uses include:

  1. Obtaining more than one foal per year out of top producing mares;
  2. Production of foals from mares in training or showing;
  3. Production of foals from late-foaling mares, leaving them open over the winter for an early start the following spring;
  4. Production of foals from mares with problems that prevent pregnancy, pregnancy maintenance, or foaling;
  5. Production of foals from some otherwise barren mares.


Whether embryo recovery and transfer takes place at an embryo transfer facility or the embryo is recovered on the farm and shipped to the embryo transfer facility, the initial process is similar.  During the breeding process the mares are examined daily to accurately determine the day of ovulation.  Seven to eight days after ovulation a catheter is inserted into the donor mare’s uterus.  The uterus is sequentially flushed with several liters of a special electrolyte/nutrient solution.  The solution is passed through a filter from which the embryo is recovered using a microscopic search.  After recovery and evaluation, the embryo is washed to remove any contaminants, placed in a nutrient media, and either inserted through the cervix and into the uterus of a specially prepared recipient mare, or shipped to an embryo transfer facility.   The first pregnancy checks are usually 5 and 7 days after transfer, which would be days 12 and 14 after ovulation.

Success rates for embryo transfer can be measured for two facets of the procedure: 1. Embryo recovery from the donor mare; 2. Establishment of pregnancy in the recipient mare.  Embryo recovery is usually the rate-limiting step in equine embryo transfer.  Recovery rates will vary with the individual mare’s fertility; young mares can have very high rates, typically around 80% per cycle.  Older mares with history of infertility, as a group, will have recovery rates around 30% per cycle.  Individuals within these groups will have higher or lower recovery rates.  Once the embryo is recovered, one can expect a pregnancy success rate of approximately 80% at 14 days, with about a 10% rate of pregnancy loss following that time.  Pregnancy rates for transported embryos are slightly lower than for on-site transfer.

Transfer Success 1998-2016


In Clinic Pregnancy/transfer % Pregnant
Grade 1 &2 2,515/2,984 84.2%
Grade 3 96/147 65.3%
Total In Clinic 2,611/3,131 83.4%
Grade 1 &2 2,381/2,898 82.2%
Grade 3 166/316 52.5%
Total Transported 2,5457/3,214 79.2%
Overall Total 5,185/6,345 81.3%