Embryonic vesicle diameter at first pregnancy diagnosis in in vivo- and in vitro-derived pregnancies and its relationship with pregnancy loss in the mare

Abstract

This retrospective study compared the relationship between embryonic vesicle diameter at first pregnancy diagnosis and early embryonic loss (EEL) across three pregnancy types: artificially inseminated broodmares (AI; n = 263), recipients carrying in vivo-derived flushed embryos (IVD-F; n = 312), and recipients carrying in vitro-produced embryos by intracytoplasmic sperm injection (IVP-ICSI; n = 161). Vesicle diameter was recorded at Day 12 post-ovulation in AI, Day 4 post-embryo transfer (ET) for IVD-F, and Day 7 post-ET for IVP-ICSI pregnancies. Mean (±SD) vesicle diameters and EEL rates were 8.1 ± 2.0 mm and 8.7% (AI), 7.2 ± 2.4 mm and 8% (IVD-F), and 8.8 ± 2.7 mm and 13% (IVP-ICSI). Small-for-age vesicles were associated with higher EEL in AI and IVP-ICSI pregnancies (P < 0.05), with the effect most pronounced for IVP-ICSI pregnancies (31.8% EEL in the 0–25th percentile vesicle size vs. 0% in the 75–100th percentile), and moderate in AI (17.6% vs. 4.6%). In IVD-F pregnancies, the difference was not significant (12.3% vs. 9.1% for the 0–25th and 75–100th percentiles). Larger vesicle size (>50th percentile) was strongly associated with IVP-ICSI pregnancy viability, including reduced fetal loss beyond Day 42 (2.5% vs. 11.3% fetal losses for >50th percentile->9mm- and <50th, respectively). In conclusion, a small-for-age vesicle at first pregnancy diagnosis is an indicator of an increased pregnancy failure risk, with the strongest prognostic value in IVP-ICSI pregnancies: 31.8% EEL for pregnancies with < 7 mm vesicles (<25th percentile size), and 2.5% EEL for > 9 mm vesicles (>50th percentile size).