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Fetal morphological features and abnormalities associated with equine early pregnancy loss

Kahler, A; McGonnell, I M; Smart, H; Kowalski, A A; Smith, K C; Wathes, D C; Mestre, A M


A Kahler

I M McGonnell

H Smart

A A Kowalski

K C Smith

D C Wathes

A M Mestre



Early pregnancy loss (EPL) occurs in approximately 8% of equine pregnancies, although the aetiology is mostly unknown and embryonic/fetal morphological abnormalities associated with EPL are not defined.


To compare the morphology of EPL to clinically normal embryos/fetuses and previously described embryonic/fetal developmental milestones. To identify morphological abnormalities associated with equine EPL.

Study design

Observational case‐control study.


Embryos/fetuses were obtained from clinically normal Thoroughbred and pony pregnancies (n = 11) and following EPL from Thoroughbred mares (n = 27). The crown‐rump length (CRL) of embryos/fetuses was measured and macroscopic morphology and developmental age were determined independently by three blinded examiners. Sagittal sections of EPL (n = 13) and control (n = 6) embryos/fetuses were assessed microscopically. Fisher's exact test was used to determine significance (P < .05) and correlations were expressed by Pearson coefficient.


Age and CRL were strongly positively correlated in clinically normal Thoroughbred and reference (n = 15, R = .9 (95% CI: 0.8‐1.0), R2 = .9, P < .0001) but not EPL embryos/fetuses (n = 19, R = .1 (95% CI: −0.4 to 0.5), R2 = .01, P = .75). Relative to controls, the CRL of EPL embryos/fetuses was smaller, with evidence of intrauterine growth retardation (IUGR) in 3/8 fetuses assessed. In 9/13 EPL embryos/fetuses, nonspecific neural tissue alterations were identified including disruption of developing pros‐, mes‐ and rhombencephalon and the presence of haemosiderin, indicating premortem haemorrhage. Failed neural tube closure was identified in 1/13 EPL embryos/fetuses. Subcutaneous haemorrhage was present in 14/27 EPL embryos/fetuses.

Main limitations

Autolysis significantly affected 15/27 EPL embryos/fetuses, excluding them from complete assessment. The IUGR reference cut‐off values were based on a small number of controls.


Morphological features associated with equine EPL were a mismatch between embryonic/fetal size and age, and alterations of the developing neural tissue and localised subcutaneous haemorrhage. Failed neural tube closure was confirmed as a rare specific abnormality.


Kahler, A., McGonnell, I. M., Smart, H., Kowalski, A. A., Smith, K. C., Wathes, D. C., & Mestre, A. M. (2020). Fetal morphological features and abnormalities associated with equine early pregnancy loss. Equine Veterinary Journal,

Journal Article Type Article
Acceptance Date Aug 31, 2020
Publication Date Sep 19, 2020
Deposit Date Oct 7, 2020
Publicly Available Date Oct 7, 2020
Journal Equine Veterinary Journal
Print ISSN 0425-1644
Publisher Wiley
Peer Reviewed Peer Reviewed
Keywords Equine, General Medicine
Public URL
Related Public URLs
Additional Information Additional Information : Associated dataset available at:


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