Physical Interplay between Equine Fetus and Uterus from Day 180 to End of Pregnancy^☆☆

Traveling of the fetal-amniotic unit throughout the uterus ceases on ∼ Day 180 followed by closure of each uterine horn. By mean Day 240, the fetus and nearly all of the pool of allantoic fluid are confined to the uterine body. Intrauterine fetal-location changes end, but in-place activity of limbs, head, and body and changes in fetal recumbency and presentation continue, sometimes vigorously. Preference for cranial presentation (fetal sternum toward maternal cervix) has been hypothesized to be stimulated by ∼ 40° incline of uterine body toward the cervix. The uterine body expands forward, but the closed uterine horns are held more closely at the tips and become perpendicular to the cranial uterine body. After closure of horns, both hind limbs enter the umbilical-cord horn apparently guided by dorsal recumbency (fetal spine toward uterine floor), close proximity of hind hooves to horn entrance, and a thick covering of each main umbilical vessel by Wharton's jelly. The limb-encased horn then begins to lie on the upper surface of the uterine body from flexure of the hind limbs. The active fetal rump may raise off the uterine floor so that the hooves of the hind limbs reach the area above the cervix. Dorsal fetal recumbency is anchored by the horn-encased hind limbs, but when the uterus rests on the mare's ventral abdominal wall, the loosened suspensory ligaments allow more rotational freedom. During parturition, the fetal head and withers twist toward the mare's spine, and the rear follows like a corkscrew.