Abnormal cleavage tied to early loss in equine IVP embryos

Abnormal cleavage patterns may give equine reproduction teams an earlier warning that an in vitro-produced embryo is less likely to hold a pregnancy. In a retrospective study published online ahead of print in Equine Veterinary Journal on July 31, 2025, investigators from UC Davis and Burns Ranch reported that equine IVP embryos showing abnormal cleavage during the first mitotic division had markedly poorer outcomes after transfer, including a total early pregnancy loss rate of 53.3% versus 22.6% for embryos with normal cleavage patterns. Earlier vitrification, reflecting faster progression to blastocyst formation, also tracked with better early pregnancy results. (pubmed.ncbi.nlm.nih.gov)

The work addresses a familiar problem in equine assisted reproduction: IVP has expanded quickly, especially with intracytoplasmic sperm injection, but embryo selection remains less standardized than many clinicians would like. The study authors note that equine IVP embryos are still associated with higher early pregnancy loss than in vivo-derived blastocysts. UC Davis’ Veterinary Assisted Reproduction Laboratory describes IVP as a growing service for producing high-value pregnancies and says the lab uses non-invasive time-lapse monitoring to define cleavage timing, morula formation, and blastocyst development. (pubmed.ncbi.nlm.nih.gov)

In the study, the team reviewed archived time-lapse material and clinical records, analyzing 70 transferred embryos with known outcomes and 114 embryos that arrested during development. Logistic regression showed that abnormal cleavage patterns both lowered the odds of pregnancy at day 14 and increased the odds of early pregnancy loss assessed through days 25 and 42. The authors concluded that cleavage behavior during first mitosis, along with time to blastocyst formation, could serve as practical classification criteria for embryo selection before transfer. Those findings are directionally consistent with larger equine IVP outcome data showing that slow in vitro embryo development reduces the chance of pregnancy after transfer. (pubmed.ncbi.nlm.nih.gov)

That conclusion fits with a broader push in equine IVP to move beyond static morphology alone. UC Davis says its imaging platform captures brightfield images every five minutes over the seven- to 10-day culture period, allowing embryologists to annotate cleavage and other developmental events without repeatedly disturbing embryos. In a 2025 UC Davis Horse Report overview, the program also highlighted embryo monitoring as a way to predict which embryos are more likely to progress appropriately and flagged chromosome errors as a contributor to early pregnancy loss in a meaningful share of cases. (vetart.vetmed.ucdavis.edu)

Industry context suggests why that matters. UC Davis reports typical ICSI cycle metrics of about 66% oocyte maturation, 66% cleavage after sperm injection, and 34% blastocyst rate. In the same Horse Report, the university said roughly 30% of fertilized oocytes develop into embryos within seven to nine days, underscoring how much value is attached to each transferable embryo. Separate retrospective data from 2,292 transfers of IVP blastocysts suggest the system can be highly productive overall: 85% of successful IVEP sessions resulted in at least one pregnancy, with that figure rising from 78% when only one blastocyst was produced to 91% with two and 98% with three or more. In that analysis, slow embryo development, poor embryo grade after thawing, advanced donor mare age, and embryo transfer later in the breeding season all negatively affected pregnancy rate, while outcomes improved over time. (vetart.vetmed.ucdavis.edu)

That larger dataset also adds nuance to how clinicians think about biologic contributors to success. Variation in pregnancy rates between most individual donor mares and stallions was relatively low, around 20%, suggesting identity alone usually has limited impact. But for some underperforming mares, the likelihood of pregnancy differed by as much as 50% depending on which stallion was used for ICSI. In other words, embryo-intrinsic markers such as cleavage pattern and developmental speed may be especially useful because they capture risk that is not fully explained by mare or stallion identity alone, while still leaving room for mare-stallion interaction effects in selected cases.

Why it matters: For veterinarians running or referring into equine embryo programs, this study supports using morphokinetic data as another decision layer when selecting embryos for transfer or freezing. It may help refine recipient mare allocation, improve discussions with pet parents about expected pregnancy risk, and reduce avoidable losses in programs built around scarce semen, older donor mares, or high-genetic-value matings. It also reinforces that embryo-related factors, not just donor mare, stallion, or recipient management, deserve close attention when early losses occur. At the same time, the broader IVP literature suggests clinicians should interpret embryo-level findings alongside other practical predictors already known to affect pregnancy odds, including donor mare age, post-thaw embryo grade, season, and in some mares, the specific stallion used. (pubmed.ncbi.nlm.nih.gov)

There’s still an important caveat: this was a retrospective analysis, and the authors note that more transfers could reveal additional interactions among morphokinetic variables. The separate donor mare/stallion study also assessed between-animal variation descriptively because of sample-size limits for that question. So while the signal is strong, the field will likely want prospective validation, clearer cutoffs for what constitutes an actionable abnormal cleavage pattern, and evidence that adopting these criteria improves live foal outcomes, not just early pregnancy endpoints. (pubmed.ncbi.nlm.nih.gov)

What to watch: Watch for larger validation studies, commercial lab adoption of time-lapse-based grading systems, and possible integration of morphokinetic findings with genetic screening and other embryo quality markers in equine IVP workflows. It will also be worth watching whether future protocols formally combine cleavage-based selection with established clinical predictors such as donor mare age, embryo grade after thawing, timing within the breeding season, and mare-stallion pairing for mares with inconsistent results. (pubmed.ncbi.nlm.nih.gov)