Equine Embryo Mobility. A Friend of Theriogenologists.

Equine embryo mobility and cessation of mobility (fixation) provide explanations to several enigmas in reproductive biology of the pregnant mare and provide an efficient solution to the twinning problem, the bane of brood-mare owners. Embryo mobility is maximum on Days 12 to 15 (Day 0 = ovulation) while the spherical embryo is growing from 9 to 23 mm in diameter. During mobility, the embryo can be anywhere in the uterine lumen regardless of side of ovulation. Mobility solved the enigmas of how a small embryo can block luteolysis in a relatively massive uterus and why the side of ovulation does not determine the side of the initial placental attachment. Fixation occurs on ∼ Day 16 at a bend or flexure in a uterine horn that has a cross sectional diameter of the endometrium that is similar to diameter of the embryo. The occurrence of fixation in the horn with smaller diameter solved several enigmas involving side of fixation such as (1) greater frequency of postpartum fixation in the formerly nongravid horn and (2) later fixation in a horse than in a pony; horses and ponies have a similar embryo diameter but horses have a larger uterus. Unilateral fixation of twins is associated with a high frequency (e.g., 85%) of natural embryo reduction (elimination of one member of a twin set) whereas bilateral fixation precludes natural embryo reduction. The theriogenologist can efficiently solve the twinning problem by compressing one mobile or bilaterally fixed embryo with finger/thumb or with the ultrasound probe.