ROLE OF THE UTERUS IN ALLANTOIC FLUID SHIFTS AND FETAL MOBILITY IN MARES

Clenbuterol was used to inhibit myometrial contractions in mares (n = 10) on Days 67-84 of pregnancy (ovulation = Day 0). Transrectal ultras onography was used at 5-min intervals for 1 h to monitor the diameter of the allantoic fluid compartment and fetal intrauterine location (um bilical cord horn, non-cord horn, uterine body), presentation, and rec umbency. The percentage of ultrasound scans in which the cornual segme nts were fully constricted (no detectable allantoic fluid) was less (P <0.0001) after clenbuterol treatment (2.2 +/- 1.1%) than before treatm ent (14.0 +/- 2.6%). The diameter of the allantoic fluid compartment w as greater (P<0.0001) after treatment (45.8 +/- 2.7 mm) than before tr eatment (36.8 +/- 2.7 mm). There were no differences between pre- vs. post-treatment examinations in the mean number of changes per hour in fetal location, presentation or recumbency. Transcervical videoendosco py was used on Days 74-84 to monitor fetal activity and mobility and d iameter of the allantoic fluid compartment at the cornual entrances in two groups (fetus treated with succinylcholine, n = 5; controls, n = 6). The mean number of fetal changes per 30 min was reduced (P<0.05) i n the treated group for location (0.6 +/- 0.4 vs. 2.8 +/- 0.7), presen tation (1.0 +/- 0.5 vs. 5.3 +/- 1.3), and recumbency (1.0 +/- 0.6 vs. 13.6 +/- 0.7). There was no difference between groups in the number of changes in diameter of the cornual entrances. Results indicated that during the equine early fetal stage: (1) the uterus is extremely activ e, resulting in pronounced and dynamic shifts in allantoic fluid distr ibution; (2) fetal mobility and changes in fetal presentation and recu mbency are dependent primarily on vigorous movements by the fetus rath er than on uterine contractions; (3) allantoic fluid shifts and fetal activity and positional changes are primarily independent phenomena.