Recent studies on equine fetal kinetics and the physical interactions between the uterus and fetal-amniotic unit are reviewed. Frequent, transient, and segmental uterine constrictions with expulsion of allantoic fluid occurred during Months 2 to 4, and complete closure of both uterine horns occurred commonly during Months 4 to 7. The noncord horn was completely closed during the last few months of pregnancy. Fetal activity began with subtle head nods on mean Day 40 and progressed to appendage and whole body movements, so that by Day 48 fetal activity was capable of raising the fetus off the floor of the allantoic sac. The percentage of time the fetus was active increased over Months 2 to 4, and the fetus displayed muscular activity an average of approximately 2 minutes during a 5-minute examination for the remainder of pregnancy. Astounding feats of mobility of the fetal amniotic unit accompanied by presentation, recumbency, and intrauterine location changes were viewed by transrectal ultrasonography, especially during Months 3 and 4. Major presentation changes (direction of fetus) and location changes between uterine horns or between a horn and the uterine body occurred a mean of 5 times per hour. The percentage of cranial presentations (front of fetal torso toward cervix) increased over Months 5 to 9 with continuous cranial presentation during Months 9 to 11. Final presentation occurred while the fetus was confined to the uterine body by closure of the two horns. Thereafter (Months 7 to 9), the fetal hind limbs progressively entered the otherwise closed umbilical-cord horn, reached the horn tip by Month 9, and remained in the horn until parturition. The apposition between the limbs and the endometrial-placental folds of the horn was close during Months 7 to 9; placental fluid did not reenter the horn during this time. However, some fluid (less-than-or-equal-to 5 mm height) was found in the limb horn in 25% and 63% of examinations during Months 10 and 11, respectively. Once the hind limbs are trapped in a uterine horn, the fetus cannot normally return to a potentially disastrous caudal presentation, despite the relatively large uterine body. In addition, entrapment of the limbs may serve to protect the umbilical cord. The hind limbs do not exit the uterine horn until parturition, but only after much of the foal has passed through the vulva.
