backend=biber, style=alphabetic, sorting=ynt ]biblatex A 4-year-old, 6-month pregnant, Welsh pony mare presented with severe acute clinical signs including depression, anorexia, pale mucous membranes, tachycardia with a grade 4/6 holosystolic murmur, and moderate expiratory dyspnoea with abnormal lung sounds. There was no evidence of placentitis or foetal distress. Electrocardiography showed sinus tachycardia. Echocardiography revealed hypertrophy of the atria and ventricles, dilatation of the pulmonary artery, and irregular hyperechoic thickening of the mitral valve leaflets, with moderate to severe regurgitation. Systolic function was severely impaired and pulmonary radiographs were consistent with pulmonary oedema. Congenital mitral valve dysplasia or early degenerative valve disease was suspected. Left heart failure did not respond to emergency treatment, and the pony developed severe jugular venous distension and a retrograde pulse. Due to the poor prognosis, euthanasia was elected. Post-mortem examination found thickening of the mitral valve leaflets and severe dilatation of all four cardiac chambers, with no other cardiac lesions. Histological examination showed myxoid valvular degeneration, but could not clearly differentiate congenital dysplasia from the development of early degenerative mitral valve disease. In this case, the physiological effect of gestation may have exacerbated the deleterious haemodynamic effect of mitral valve insufficiency.