equine pregnancy rates
Laboratory-, or in vitro-,produced embryos don’t look like embryos that are flushed out of a horse’s uterus. But they actually appear to survive the freezing process better than those “natural or flushed” equine embryos. And given the right conditions, they can lead to pregnancy rates of approximately 70%.

“The good thing about these in vitro embryos is that they’re very small compared to flushed embryos, and they don’t have that embryonic capsule,” fluid that causes issues with cryopreservation (freezing), said Anthony Claes, DVM, PhD, Dipl. ACT, assistant professor in equine reproduction at the Utrecht University Faculty of Veterinary Medicine, in the Netherlands.

Claes and colleagues produced 261 equine in vitro embryos using intracytoplasmic sperm injection (ICSI) and cultured them up to the blastocyst stage. Then, they froze (cryopreserved) them and stored them—some for up to two years, he said. Later, they thawed the embryos and implanted them in recipient mares at four, five, or six days after ovulation. Finally, the researchers investigated other factors that might influence pregnancy success, such as mare age and management, season, and more.

Overall, they found 56% pregnancy rate up to 37 days after implantation. However, of those, 16% failed before 42 days of gestation, he said.

Surprisingly few factors appeared to be linked to the success or failure of pregnancy among those they studied, Claes added. The year the embryo transfer occurred had an impact on the pregnancy rate, he said. And, pregnancy rates were higher when embryos were transferred to the recipient on Day 4 or 5 after ovulation compared to Day 6 after ovulation.

Claes said the study spanned the “early days” of the team’s clinical OPU/ICSI program—2015 and 2016. “Since then, they’ve we decided to try Day 4 with our in vitro embryos, and the rates just started going up and up. So it looks like these laboratory-produced embryos do much better in a less advanced uterine environment.”

Focusing on this day has significantly enhanced the rate of positive results, he added.

Additionally, the team found, if in vitro embryos develop especially slowly, they’re more likely to result in embryonic loss after the initial pregnancy check, according to Claes. Therefore, these small for age pregnancies might be an early indicator of impending pregnancy loss.

These in vitro embryos are frozen when they’re about eight days old—but that doesn’t mean they’re “eight days old” in embryo terms, Claes noted. Because they grow in different conditions than natural embryos, they appear to develop differently  and could actually be closer to the developmental stage of 5 or 6  days even when they’re technically eight days old.

“We don’t really know what all the details are at the moment,” he said. “We just know that they’re not the same as flushed embryos and can’t be treated in the same way.”

The study, “Factors affecting the likelihood of pregnancy and embryonic loss after transfer of cryopreserved in vitro produced equine embryos,” was published in the Equine Veterinary Journal.