Dynamics of uterine diameter and endometrial morphology during the estrous cycle and early pregnancy in mares

Abstract

Transrectal ultrasonography was used in mares to characterize changes in uterine size and endometrial echotexture during the interovulatory interval (Experiment 1; n = 15 interovulatory intervals) and early pregnancy (Experiment 2; n=6). Uterine horn diameters were measured at specific locations which were identified by surgically attached, hyperechogenic markers. Ultrasonic endometrial echotexture was scored on a scale of one to tour based on the degree of endometrial folding (one is minimal; four is maximal). In Experiment 1, the mean length of the interovulatory interval was 23 days. Mean uterine horn diameter (mm) averaged over both horns in nonbred mares was greater ( P < 0.05) on each of Days 16–22 (range of means 30.1 ± 0.7–31.7 ± 0.7) than on each of Days 1–14 (means 24.1 ± 0.4–26.6 ± 0.6). Horn diameters gradually decreased ( P < 0.05) between Days 0 and 11 and increased ( P < 0.05) between Days 11 and 21. The presence of a corpus luteum or a preovulatory follicle had no differential effect on the diameter of the ipsilateral horn. The mean endometrial echotexture score was greater ( P < 0.05) on each of Days 15–22 (means 2.2 ± 0.1–2.9 ± 0.2) than on any other day of the interovulatory interval (means 1.0 ± 0.0–1.6 ± 0.1). Mean echotexture scores decreased ( P < 0.05) between Days 0 and 1 and increased ( P < 0.05) between Days 12 and 17. In Experiment 2, the mean horn diameter in pregnant mares on each of Days 17–30 (means 20.9 ± 0.4–23.5 ± 0.8) was smaller ( P < 0.05) than on each of Days 0–15 (means 24.9 ± 0.5–29.4 ± 0.9), except for Day 12; horn diameter gradually decreased ( P < 0.05) between Days 0 and 25. No effects on horn diameter over Days 0–30 were found when the horns were designated on the basis of side of fixation of the embryonic vesicle. Mean echotexture scores decreased ( P < 0.05) between Days 0 and 1 and 1 and 3 with transient increases ( P < 0.05) between Days 14 and 15 and 22 and 25. In both experiments, there was a positive temporal relationship ( P < 0.0001) between changes in uterine horn diameter and endometrial echotexture (nonbred, r = 0.75; pregnant, r = 0.31). There was also an inverse temporal relationship between observed changes in uterine diameter and reported changes in uterine tone for nonbred and pregnant mares.