Factors affecting the likelihood of pregnancy and embryonic loss after transfer of cryopreserved in vitro produced equine embryos.

This research studied the factors affecting the success of pregnancy and embryonic development in horses after the transfer of in vitro produced (IVP) embryos that were frozen and then thawed. The results showed that the success rates did not significantly vary based on the age and reproductive history of the donor mare, the age and reproductive status of the recipient mare, or the interval from sperm injection to blastocyst formation, among other things. However, the likelihood of pregnancy tended to be lower when embryos came from infertile mares or when they were transferred into the recipient on the sixth day after ovulation.

Methodology

• The study was a retrospective clinical one, involving 261 blastocysts that were produced from immature oocytes of Warmblood mares.
• These embryos were frozen and stored, then thawed and transferred into recipient mares on the fourth, fifth, or sixth day after the mares’ ovulation.
• The researchers examined a range of factors to see if they had any bearing on the success of the pregnancy or the development of the embryo. These factors included attributes of both the donor and the recipient mares and various aspects of the embryos and their management.

Results

• After the embryo transfer, pregnancy rates were 56%, 49%, and 48% at the 7-10, 23 and 37-day points, respectively.
• Further, the study found that the developmental stage of the IVP embryos after being transferred was equivalent to day 5 or 6 embryos that had been fertilized in the mare’s body (in vivo).
• Of all the factors studied, the only one that influenced the likelihood of pregnancy and embryonic development was the year of transfer.
• However, there was a trend towards lower success rates when the source of the embryo was an infertile mare, or when the embryo was transferred into the recipient on the sixth post-ovulation day.

Conclusion

• The research concluded that, barring a few exceptions, the factors studied did not significantly affect the likelihood of pregnancy or embryonic development after IVP embryo transfer.
• The study recommended that the IVP embryos be transferred into recipient mares on the fourth or fifth day after ovulation.
• The researchers also suggested that a slower rate of expansion of the embryonic vesicle in the week following the embryo transfer indicates a higher risk of subsequent loss of the embryo.