Pregnancy and Parturition - Horse Anatomy

Pregnancy

Gestation length in the mare is approximately 330-340 days, however it is usually consistent within the individual mare from one pregnancy to the next.

Maternal Recognition of Pregnancy

The presence of a conceptus prevents luteolysis. In the presence of a conceptus, endometrial production of PGF2α is significantly reduced. The conceptus must migrate within the uterus from one uterine horn to the other 12-14 times a day during days 12, 13 and 14 of gestation in order to inhibit PGF2α. This migration is necessary, as the equine conceptus does not elongate, so there is less contact between the conceptus and the endometrial surface. It must move to distribute pregnancy recognition factors to the endometrial cells. The conceptus does produce proteins to effect the recognition of pregnancy, but specific roles are unknown. The luteolysin in the non-pregnanct cycle is thought to be PGF2α.

The embryo takes 6 days to traverse the oviduct (in other species this is normally ~4 days). It stops at various spots in the uterus, spending 5-20 minutes in each. The conceptus remains spherical in shape.

• Day 6-22: the trophectoderm secretes a glycocalyx, which hardens to form a capsule. This prevents attachment of the embryo to the uterine endometrium.
• Day 7-17: Peristaltic contractions of the uterine myometrium move the embryo around the uterus. The conceptus begins to secrete oestrogens, but their role is unknown. If pregnant, upregulation of oxytocin receptors between day 10-16 is inhibited
• Day 17: The myometrium clamps the embryo in position at the base of the uterine horns, preventing movement.