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

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 (P50 th percentile) was strongly associated with IVP-ICSI pregnancy viability, including reduced fetal loss beyond Day 42 (2.5% vs. 11.3% fetal losses for >50 th percentile->9mm- and 9 mm vesicles (>50 th percentile size). • Embryonic vesicles within the 0-25 th percentile are at greater risk of death • There is no relationship between IVD-F embryo vesicle diameter and EEL • IVP-ICSI embryo vesicles ≥9 mm in diameter 7 d post-ET have only 2.5% risk of EEL