Oocyte Transfer in Mares

Oocyte transfer can help in getting foals from older mares.

A single oocyte or egg is present in every ovarian follicle. In the normal progression of reproduction, an egg is released from the follicle at ovulation and is picked up by and transported down the oviduct. If the mare was bred or inseminated, several thousand spermatozoa would be present and waiting within the oviduct. A single spermatozooan may locate, penetrate and fertilize the egg. The developing fertilized egg or embryo spends the next five to six days within the oviduct before entering the uterus. For a pregnancy to occur, mares have to develop a large follicle and ovulate, the egg must be fertilized within the oviduct and passed into the uterus, and the uterine environment must be sufficiently healthy to allow for embryonic development.

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If any of these factors fail or are inadequate, the mare will not become pregnant or will not stay pregnant. The technique of oocyte or egg transfer bypasses issues of ovulation failure or severe oviduct, uterine or cervical problems. Oocyte transfer may be successful in obtaining foals from mares that cannot carry their own pregnancies or cannot donate an embryo for transfer to a recipient. The technique is most often used in older mares with a history of subfertility or infertility. Oocyte transfer has also been used to obtain foals from eggs harvested from the ovaries of mares that have died.


Traditional oocyte transfer involves harvesting of one or more eggs from the preovulatory follicles of mares in estrus. Follicular development in an oocyte donor mare is closely monitored by ultrasonography. Human chorionic gonadotropic or deslorelin acetate is administered to the donor mare once a large follicle is detected to help mature the follicle and egg. The most common technique used to harvest an egg is transvaginal, ultrasound-guided follicular aspiration. The donor mare is sedated and a special ultrasound probe and guide are inserted into the vagina and positioned against the front vaginal wall adjacent to the cervix. A long needle, specifically designed for egg collection, is passed down the guide. The needle penetrates the vaginal wall and into the ovary containing the pre-ovulatory follicle. The follicular fluid and oocyte are aspirated from the follicle and collected in a sterile container.

Eggs may be transferred immediately into a surrogate or recipient mare, or the egg may be cultured in an incubator for 12 t o16 hours prior to transfer. Recipient mares may be either cycling mares synchronized with the donor mare or may be noncycling mares treated with estrogen and progesterone to mimic a natural cycle. If a cycling recipient mare is to be used, it is critical to remove the egg from her preovulatory follicle to ensure that any pregnancy that develops after transfer is from the donor oocyte. The donor egg is transferred into the oviduct of the recipient mare via a flank surgery. The recipient mare is inseminated with spermatozoa from a fertile stallion either before egg transfer, directly after transfer or both. The stallion is selected by the owner of the egg donor mare, and fresh, cooled or frozen semen may be used.

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Fertilization, embryonic development and all of gestation occur within the recipient mare. It is common for the recipient mare to be supplemented with progesterone or a synthetic progestin during the first few months of pregnancy.


The factors that most strongly influence success of oocyte transfer are age and health status of the donor mare and quality of semen provided for the procedure. As with embryo transfer, parentage must be verified through genetic testing of the foal, sire and egg donor.

If you're interested getting more information about these types of procedures, check out our "Embryo Transfer" article, which explains in easy-to-read language how the embryo transfer process works!