Equine fetal kinetics: entry and retention of fetal hind limbs in a uterine horn

Transrectal ultrasonic examinations were made in 31 pregnant pony mares once a week during Months 6 to 11. Each uterine horn was divided into 3 approximately equal segments (caudal, middle, cranial). The percentage of examinations with cranial fetal presentation increased (P < 0.05) progressively from 58% at Month 6 to 99% at Month 9 and was followed in all mares by entry of the fetal hind limbs into one uterine horn. The mean number of uterine-horn segments with limb parts increased (P < 0.05) between each set of consecutive months from Month 6 to Month 10. Initially (Months 7 and 8), retraction of limbs after entry into the caudal and middle segments of the horn was common (31% incidence). The mean day of final entry of the limbs without detection of subsequent retraction was Day 230 +/- 2.2. Both uterine horns were closed during the examination preceding final entry of the hind limbs into one horn in 25 of 29 (86%) mares. The limbs reached the cranial segment in most examinations by Months 9 and 10 (73% and 98%). The cross-sectional height, as seen on the ultrasound screen, of both uterine horns increased (P < 0.05) progressively during Months 7 to 10. Between Months 10 and 11, the height of the horn containing the limbs decreased (P < 0.05); this result was attributable to a flattening of the horn and thinning (P < 0.05) of the horn wall above a hoof and metatarsal bone. Apparent placental fluid (nonechogenic areas > 5 mm in height) between the hind limbs and the cornual wall was detected in 0, 10, 25, and 63% (P < 0.01) of examinations for Months 8 to 11, respectively. Results indicated that entry of the fetal hind limbs into a uterine horn was initially tentative and became final during a mean of Month 8. The apposition between the cornual wall and limbs was close during Months 7 to 11. By Month 11, the fetal-limb horn became flatter and thin-walled and usually contained placental fluid.