Incidence and nature of disorientation of the embryo proper and spontaneous correction in mares

Orientation of the embryo proper at the periphery of the equine embryonic vesicle is normally antimesometrial or on the ventral aspect of the embryonic vesicle (6 oʹclock relative to 12 oʹclock at the center of the mesometrial attachment). An early ultrasonographically detectable vascular endometrial indicator of the future position of the embryo proper has been reported previously and was first detected in a mean 2.5 days before detection of the embryo proper. In the current study, four occurrences of disorientation of the embryo proper were found in a group of 30 mares (incidence, 13%). When first detected, the early indicator of the clock-face position of the embryonic pole for the disorientation and normal orientation group, respectively, was 1.3 ± 0.3 and 0.4 ± 0.1 hours from the 6 oʹclock position (P < .008). The extent of disorientation increased progressively over days 16 to 19, so that the embryo proper was at 3, 9, 9, or 10 oʹclock. Disorientation was associated with a flaccid uterus and defective encroachment of the dorsal endometrium on the vesicle in three of the four mares. In a second study, disorientation of the embryo proper occurred in two mares with apparently normal uterine tone and endometrial encroachment. When first detected, the embryo proper was at 9 or 10 oʹclock. However, asymmetric enlargement of the allantoic sac spontaneously corrected the disorientation, so that orientation of the umbilical cord attachment was at a normal position near 12 oʹclock.