Equine fetal kinetics: Allantoic-fluid shifts and uterine-horn closures

Uterine filling by the conceptus and uterine constrictions and allantoic-fluid shifts after filling were studied by daily ultrasound examinations on Days 40 to 74 in 10 mares. Allantoic fluid was first detected at the extremities of the uterus on the following days: 1) tip of umbilical-cord horn, Day 60.0 +/-1.9 (mean +/-SEM); 2) tip of noncord horn, Day 64.8 +/-1.3; and 3) near cervix, Day 48.5 +/-1.3. Transient uterine constrictions with expulsion of allantoic fluid were detected in 37% of 271 examinations. Other end points were studied by weekly ultrasound examinations from Day 40 to term. Height of cross-sections of the fetal body increased (P<0.01) progressively over Monthes 2 to 8, whereas the height of placental fluid minus fetal body decreased (P<0.05) after Month 3. An increasing (P<0.05) frequency of examinations with a reclosed uterine horn occurred between Months 3 and 4. The frequency of reclosed noncord horns continued to increase (P<0.05) after Month 4 and reached 96% at Month 7. The frequency of reclosure of cord horns plateaued (no significant differences) at 54% over Months 4 to 7 and then rapidly decreased (P<0.05). Both uterine horns were closed during the examination that preceded the permanent entry of the fetal hind limbs into the cord horn in all of 9 mares in which this temporal relationship was studied. After entry of the fetal hind limbs, the cord horn remained constricted upon the limbs until at least Month 10. The results provide rationale for the following hypotheses: 1) The gradual decrease in fetal mobility (location and presentation changes) previously reported for Months 4 to 8 is associated with growth of the fetus and a concomitant decrease in volume of allantoic fluid and number of open uterine segments; 2) Closure of both uterine horns near mid-pregnancy plays a role in final selection of cranial fetal presentation followed by entry of the fetal hind limbs into the cord horn.