Fixation and orientation of the early equine conceptus.

Fixation and orientation of the conceptus within the uterine lumen of 40 barren mares were examined by ultrasound daily on days 11-21 and at three-day intervals thereafter until day 48. The growth curve of the in situ conceptus had a distinct plateau between days 17-24, as determined by measurements of the width and area of conceptuses on the ultrasound images. The vesicle expanded at an average daily rate of 3-4 mm before the plateau and 2-3 mm after the plateau. Dramatic changes occurred in the shape of conceptus. The predominant shapes were approximately as follows: days 11-16, spherical; day 17, oblong; days 18-21, triangular; and days 24-48, irregular. The embryo was detected in some mares on day 20 and in most mares by day 21. When first detected, the embryo was always on the ventral aspect of the vesicle, opposite to the mesometrial attachment. The embryo gradually moved toward the mesometrial attachment over days 21-36 in association with the growth of the allantois beneath the embryo. It was hypothesized that: 1) vesicle fixation (cessation of mobility) was due to increasing uterine tone with impingement of the endometrial folds upon the vesicle, and 2) vesicle orientation was due to greater strength of the yolk sac wall at the embryonal pole coupled with greater impingement upon the vesicle by the dorsal uterine wall than by the ventral wall.