Embryonic vesicle diameter and its relationship with pregnancy loss in the mare

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

Anim Reprod Sci. 2026 Mar 18:289:108173. doi: 10.1016/j.anireprosci.2026.108173. Online ahead of print.
Authors
Juan Cuervo-Arango 1 , Tom A E Stout 2 , Anthony N Claes 3
Affiliations

1 Equine Fertility Group, Facultad de Veterinaria, Universidad Cardenal Herrera-CEU, CEU Universities, Alfara del Patriarca, Spain. Electronic address: juan.cuervo@uchceu.es.
2 Department of Veterinary Science, University of Kentucky, Lexington, KY, USA.
3 Utrecht University, Department of Clinical Sciences, the Netherlands.

PMID: 41861690
DOI: 10.1016/j.anireprosci.2026.108173

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).

Keywords: Embryonic death; Embryonic vesicle diameter; Horse; ICSI; Pregnancy diagnosis.

Copyright © 2026 The Authors. Published by Elsevier B.V. All rights reserved.