Abnormal cleavage patterns in equine in vitro-produced embryos lead to higher early pregnancy loss

Despite significant advances, in vitro production (IVP) of equine embryos continues to lack standardised embryo classification criteria and is associated with increased rates of early pregnancy loss compared with in vivo-derived blastocysts.

To evaluate morphokinetic characteristics of the first mitotic division and early embryonic development in IVP blastocysts and their association with embryo development, as well as pregnancy rate and early pregnancy loss following embryo transfer.

Retrospective analysis of archived material and clinical records.

We retrospectively analysed morphokinetic characteristics of transferred IVP embryos with known pregnancy outcomes as well as those from arrested embryos. We analysed time-lapse images of 70 transferred embryos and 114 arrested embryos to identify and compare the frequency of abnormalities during the first mitotic division (cleavage patterns) and recorded the time to vitrification or arrest. A logistic regression model with a logit link was used to evaluate pregnancy success at 14 days and early pregnancy loss (EPL) at 25 and 42 days in relation to the recorded morphokinetic characteristics in SAS.

Earlier vitrification (which corresponds to the time to blastocyst formation) increased the odds of pregnancy at 14 days and decreased pregnancy loss until 25 days, but not between 25 and 42 days. Abnormal cleavage patterns decreased the odds of pregnancy and increased the odds of EPL, with embryos exhibiting abnormal cleavage patterns showing a 53.3% total EPL rate compared with 22.6% for normal cleavage patterns (p < 0.05).

While our analysis provides sufficient statistical power to draw conclusions regarding the main objectives, increasing the number of embryo transfers could reveal additional interactions among morphokinetic characteristics that influence pregnancy success.

Overall, we demonstrated a relationship between cleavage patterns during the first mitotic division, time to blastocyst formation, pregnancy rate and early pregnancy loss risk for equine IVP embryos. These features can serve as a classification method to identify and select embryos with higher pregnancy potential for transfer, contributing to reducing the gap in pregnancy rates between in vivo and in vitro-produced embryos.