Transrectal ultrasonography of the caudal placental pole and fetal eyeball diameter and associations with the season, duration of gestation, placental weight, sex of the foal, and birthweight in donkeys☆

This study aimed to establish normal ranges and determine variables affecting the duration of gestation, placental and fetal eyeball measurements in donkeys. Jennies (n = 121) were submitted to transrectal ultrasonography at 15-day-intervals from 120 days of gestation until parturition. During each examination, variables assessed included: the combined thickness of uterus and placenta (CTUP), fetal eyeball diameter, amniotic membrane thickness, allantoic and amniotic fluids echogenicity, and chorioallantois edema. Parametric data were analyzed with t-test, linear regression, mixed models, ANOVA, Tukey's as posthoc, and non-parametric data with Kruskal-Wallis and Dunn's. Statistical significance was set at P < .05. The mean duration of gestation was 365.4 ± 10.4 days, and there were effects of the season (P < .001), sex of the foal (P = .001), but there was no interaction (P = .89). Spring-bred jennies had 16d longer gestation than summer- and fall-bred (P < .0001), and with no difference between the latter (P = .81). Colts had four days longer gestation than fillies (P = .0001). The proportion of colts (51%) and fillies (49%) were similar (P = .8). The CTUP measurements varied with gestation age (P < .0001), but there were no effects of the season (P = .68), sex of the foal, or interactions (P = .80). The incidence and scores of chorioallantois edema increased from 240 to 300 days (P < .001). Amniotic membrane thickness increased minimally between mid- and late-gestation (P = .02). The fetal eyeball was strongly associated with gestational age (r=0.78) but not with any other variable (P > .05). Allantoic fluid echogenicity did not vary throughout gestation (P = .21); however, the amniotic fluid echogenicity increased from mid-to late-gestation (P = .0001). In conclusion, the season and sex of the foal affected the duration of gestation, but no other variables. The CTUP displayed a sigmoid increase throughout pregnancy, and gestational age was the only variable affecting it. The incidence of chorioallantois edema and scores increased from 240 to 300 days of gestation. Fetal eyeball diameter was strongly associated with gestational age. Amniotic fluid echogenicity increased late in pregnancy, but not the allantoic fluid.