Factors affecting the likelihood of pregnancy and embryonic loss after transfer of cryopreserved in vitro produced equine embryos.
Authors: Claes A, Cuervo-Arango J, van den Broek J, Galli C, Colleoni S, Lazzari G, Deelen C, Beitsma M, Stout T A
Journal: Equine veterinary journal
Summary
# Editorial Summary: Cryopreserved In Vitro Equine Embryos—Pregnancy Outcomes and Transfer Timing In vitro embryo production (IVEP) using intracytoplasmic sperm injection (ICSI) offers reproductive options for mares with compromised fertility, yet clinical data on frozen-thawed embryo survival remain limited. Researchers analysed 261 cryopreserved blastocysts from 116 Warmblood mares to identify factors influencing pregnancy establishment and embryonic loss following transfer into recipient mares, examining donor characteristics, recipient factors, embryo developmental kinetics, and management variables across multiple centres. Pregnancy rates reached 56% at days 7–10 post-transfer, declining modestly to 48% by day 37, with transferred IVP embryos developing at a rate consistent with day 5–6 in vivo embryos. Transfer timing emerged as clinically significant: embryos placed on day 4 or 5 post-ovulation showed superior outcomes compared to day 6 transfer, and smaller embryonic vesicle diameter at 7 days post-transfer correlated with subsequent pregnancy loss. Practitioners should prioritise transferring cryopreserved IVP embryos within the day 4–5 window and monitor early vesicle expansion ultrasonographically, as restricted growth signals increased miscarriage risk; donor mare fertility status also warrants consideration during case selection, despite the year of transfer being the only statistically significant variable across the cohort.
Read the full abstract on PubMed
Practical Takeaways
- •When transferring frozen IVP embryos, time the transfer to recipient mares on day 4 or 5 after ovulation rather than day 6 to optimize pregnancy rates
- •Monitor embryonic vesicle diameter at day 7 post-transfer as an early indicator of viability; slower expansion suggests higher loss risk
- •Expect pregnancy rates around 50% with cryopreserved IVP embryos, with continued loss risk through mid-gestation
Key Findings
- •56% of mares achieved pregnancy 7-10 days after transfer of cryopreserved IVP embryos, declining to 49% at day 23 and 48% at day 37
- •Transfer timing on day 4 or 5 post-ovulation showed higher pregnancy likelihood than day 6 transfer
- •IVP embryos from infertile donor mares showed reduced pregnancy likelihood
- •Slower embryonic vesicle expansion 7 days post-transfer was associated with higher embryonic loss risk