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Background: Excessive umbilical cord length (UCL) is associated with equine pregnancy loss. However, a lack of UCL reference values makes it difficult to define excessive UCL. Further, factors associated with differences in UCL are poorly understood. Objectives: To (i) report the total, allantoic and amniotic UCL in healthy term pregnancies in Thoroughbreds, (ii) describe the relationship between gestational age and UCL, fetal weight and crown rump length (CRL) using clinically normal pregnancies (CNPs) from mares dying during gestation, and (iii) identify associations between UCL and maternal age and parity, paternal age, and fetal sex. Study design: Cross‐sectional. Methods: Data including UCLs, fetal weight, CRL and maternal age, parity, paternal age and fetal sex were taken from CNPs from Thoroughbred mares dying during gestation (n = 32), and placentas from HTPs (n = 34) in England. Correlations were assessed using Spearman's rank with significant correlations estimated by locally weighted scatter plot smoothing (LOWESS). Regression plots were fitted to highly correlated variables to further assess and quantify relationships. Differences in UCL between categorical variables were assessed using Kruskall Wallis and Mann–Whitney U tests. Results: The median total, amniotic and allantoic HTP UCLs were 53.5 cm (interquartile range [IQR] 16), 29.5 cm (IQR 7) and 25.0 cm (IQR 8) respectively. Gestational age and amniotic UCL were moderately correlated (rho = 0.53, p = 0.04), with LOWESS estimating an exponential increase followed by plateauing at around Day 200. Nonlinear associations were observed between fetal weight and gestational age and CRL (adjusted r2 = 0.98 and 0.95 respectively). A linear association was observed between gestational age and CRL: predicted CRL = −17.60 + 0.38 × gestational age, p < 0.001. Main limitations: Limited availability of CNPs from mares dying during gestation. Estimated relationships can only approximate growth. Conclusions: This study provides important UCL and fetal size reference values, which may aid in assessing abnormalities. For the first time, associations between UCL and gestational age have been described. [ABSTRACT FROM AUTHOR]

Background: Pregnancy loss after Day 70 of gestation manifests as abortion, stillbirth or perinatal death. While previous studies have reported the diagnoses of laboratory submissions, none have quantified the incidence and causes of abortions, stillbirths and perinatal mortality at a population level. Objectives: To report the incidence and causes of pregnancy loss after Day 70 of gestation in a cohort of Thoroughbreds. Study design: Retrospective cohort study. Methods: Outcomes of Day 70 pregnancies were collected from eight Thoroughbred farms over the 2013‐2017 breeding seasons. Stud, veterinary and laboratory records were supplemented with publicly available data. Cause of loss was categorised using custom criteria. Results: Data were collected on 3,586 pregnancies from 1,802 mares. The incidence risk of a pregnancy failing to produce a live foal at 24 hours post parturition was 7.3% (95% confidence interval (CI) 6.5‐8.2, equating to 7.3 cases per 100 Day‐70 pregnancies). The incidence of pregnancy loss between Day 70 and 300 of gestation, Day 301‐315 and stillbirth/perinatal death was 4.0% (95% CI 3.4‐4.7), 0.3% (95% CI 0.2‐0.6) and 1.4% (95% CI 1.1‐1.9) respectively. Of the pregnancy losses where tissue was available, 61.1% were submitted for post‐mortem examination. The incidence risk of loss due to umbilical cord‐related pathologies was 1.5% (95% CI 1.1‐1.9), 0.4% (95% CI 0.2‐0.6) for noninfectious placental disease and 0.3% (95% CI 0.2‐0.6) for both infectious placentitis and Equine Herpesvirus infection. No primary diagnosis was made in 11.2% of the cases which underwent full post‐mortem examination. Main limitations: It was not possible to differentiate between intra‐partum stillbirth and early post‐partum death. Conclusion: Pregnancy loss after Day 70 of gestation is a significant source of loss in the Thoroughbred with umbilical cord‐related pathologies being the most commonly diagnosed cause. Reporting the incidence of pregnancy loss at a population level with clear case definitions will allow for accurate global comparisons. [ABSTRACT FROM AUTHOR]

Background: 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. Objectives: 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. Methods: 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. Results: 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. Conclusions: 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. [ABSTRACT FROM AUTHOR]