Abnormal cleavage may help flag higher-risk equine IVP embryos

Abnormal early cleavage appears to be a meaningful warning sign in equine in vitro-produced embryos. In a retrospective study published online July 31, 2025, in Equine Veterinary Journal, investigators reported that embryos showing abnormal cleavage patterns during the first mitotic division were less likely to establish pregnancy and more likely to be lost early after transfer. Total early pregnancy loss reached 53.3% in embryos with abnormal cleavage, compared with 22.6% in embryos with normal cleavage. (pubmed.ncbi.nlm.nih.gov)

That finding lands in a part of equine reproduction where clinicians have had growing technical capability, but still limited consensus on how best to grade embryos before transfer. The paper’s authors note that equine IVP remains associated with higher early pregnancy loss than transfer of in vivo-derived blastocysts, despite major advances in the field. Earlier work has also shown that IVP embryos can differ biologically from in vivo embryos, including altered inner cell mass organization, while other studies have linked embryo development speed and morphology with downstream foaling outcomes. A separate large retrospective EVJ study of 2292 IVP blastocyst transfers likewise found that slower in vitro development, poorer embryo grade after thawing, advanced donor mare age, and embryo transfer later in the breeding season all reduced pregnancy rates. (pubmed.ncbi.nlm.nih.gov)

For the new study, the UC Davis-Burns Ranch team reviewed archived time-lapse material and clinical records from 70 transferred IVP embryos with known outcomes, plus 114 embryos that arrested during development. They examined morphokinetic characteristics during the first cleavage division and measured time to vitrification, which served as a marker for time to blastocyst formation. Earlier vitrification was associated with better pregnancy odds at 14 days and lower pregnancy loss through 25 days, though not between 25 and 42 days. The authors concluded that both cleavage pattern and speed of development could serve as practical classification criteria for selecting embryos with higher pregnancy potential. (pubmed.ncbi.nlm.nih.gov)

The broader literature points in the same direction. A 2025 Theriogenology study found that, among post-thaw features of equine IVP blastocysts, the speed at which embryos reached the blastocyst stage was the clearest predictor of pregnancy establishment, while marked blastomere herniation reduced early pregnancy rates. And in the larger EVJ dataset on donor and stallion effects, IVEP was generally highly efficient: 85% of successful IVEP sessions resulted in at least one pregnancy, including 78% of sessions when only one blastocyst was produced, 91% when two were produced, and 98% when at least three were produced. That paper also suggested that embryo-level indicators can help veterinarians set expectations and refine recipient selection, even when they don’t necessarily lead to discarding an embryo outright. (research-portal.uu.nl)

There doesn’t appear to be much published outside commentary yet on this specific EVJ paper, but the findings fit with a wider industry push toward more objective embryo assessment. In related equine IVP research, investigators have increasingly focused on developmental speed, morphology, chromosomal competence, biopsy-based genetic testing, and management factors as ways to improve transfer success. The donor/stallion study also found that variation in pregnancy rates between most individual mares and stallions was relatively low, at about 20%, although for some underperforming mares the likelihood of pregnancy differed by as much as 50% depending on the stallion used for ICSI. Some authors have explicitly raised the possibility that slower development may reflect chromosomal or cell-lineage abnormalities, though that remains an inference rather than a settled conclusion. (research-portal.uu.nl)

Why it matters: For veterinary professionals working in sport horse, breeding, or advanced theriogenology practice, this study supports adding dynamic developmental data to embryo selection workflows when available. Static morphology at transfer has value, but first-cleavage behavior may offer earlier and more biologically relevant insight into embryo competence. That could affect how clinics prioritize embryos for transfer, counsel pet parents on expected success rates, allocate recipient mares, and evaluate whether investment in time-lapse systems is justified. It also reinforces that not all IVP embryos with acceptable appearance carry the same risk profile. And because other recent data suggest pregnancy odds are also shaped by donor mare age, post-thaw embryo quality, seasonal timing, and occasionally specific mare-stallion combinations, embryo scoring is likely to work best as one part of a broader decision framework. (pubmed.ncbi.nlm.nih.gov)

The study also matters because equine IVP is no longer niche. Clinical programs are scaling, and related research on donor mare identity, stallion effects, development speed, and post-thaw morphology suggests pregnancy outcomes are shaped by both embryo biology and management decisions. Encouragingly, the donor/stallion analysis found that the likelihood of pregnancy after transfer has improved over time and that individual mare and stallion identity usually have a fairly limited overall impact, even if certain pairings perform noticeably better or worse. As programs become more data-driven, embryo grading systems that combine cleavage pattern, developmental timing, morphology, and key clinical context may offer a more standardized path to better outcomes. (research-portal.uu.nl)

What to watch: The next step is prospective validation in larger clinical cohorts, especially to determine whether cleavage-pattern scoring improves decision-making in real time and whether journals, labs, or specialty groups move toward standardized equine IVP grading criteria. It will also be worth watching whether future scoring systems formally incorporate donor age, post-thaw grade, seasonal timing, and mare-stallion interaction effects alongside morphokinetic data. (pubmed.ncbi.nlm.nih.gov)