Equine Embryo Mobility. A Friend of Theriogenologists.

The research focuses on the movements of equine embryos and their cessation (fixation) in the uterus of a pregnant mare, and how these phenomena influence certain reproductive behaviors and provide solutions to twinning problems, common concerns for brood-mare owners.

Embryo Mobility

• The study asserts that embryo mobility is at its peak between Days 12 to 15 (where Day 0 is ovulation) when the embryo grows from 9 to 23 mm.
• During this period, the embryo can be located anywhere in the uterine lumen, irrespective of the side of ovulation.
• This phenomenon explains how a small embryo can inhibit reverse growth of the corpus luteum (which produces certain hormones necessary for pregnancy) in a considerably large uterus.
• It also clarifies why the side of ovulation doesn’t determine the initial attachment point of the placenta.

Embryo Fixation

• The research notes that embryo fixation typically happens around Day 16, at a bend in the uterine horn. This area has a cross-sectional diameter similar to the embryo’s.
• The fixation in the smaller-diameter horn explains why the non-gravid (not pregnant) horn is more likely to experience post-partum fixation, as well as why fixation occurs later in horses than in ponies. Even though horses and ponies have similar embryo diameters, horses have a larger uterus.

Twinning and Natural Embryo Reduction

• When twins fixate in the same horn (unilateral fixation), there is typically a high frequency (85%) of natural embryo reduction (like natural “miscarriage” of one of the twins).
• However, when each twin attaches to a different horn (bilateral fixation), this natural reduction process doesn’t happen.
• The research indicates that this twinning problem can be addressed by a theriogenologist (a veterinarian specializing in animal reproduction) who can manually compress one of the embryos either during its mobility phase or when bilaterally fixed.