Prevalence and effect of uterine luminal free fluid on pregnancy and litter size in bitches

Free fluid within the uterine lumen has been identified ultrasonographically as a transient finding in normal bitches in the period immediately after mating, and delayed clearance of the fluid has been documented in bitches with endometrial hyperplasia. This study used B-mode ultrasonography to investigate the prevalence of free fluid in the uterine lumen in normal bitches (n = 53) and bitches with endometrial hyperplasia (n = 10) on Days −7, 0, +5, and +14 in relation to estimated ovulation. For normal bitches without endometrial hyperplasia, 47 (86.6%) bitches become pregnant, whereas for bitches with endometrial hyperplasia, 3 (30%) become pregnant. In both the groups, the presence of uterine fluid on Days −7 and 0 was not associated with the probability of a bitch becoming pregnant, whereas the presence of uterine fluid on either Day +5 or +14 was significantly associated with a reduced likelihood of pregnancy. In pregnant bitches, uterine luminal fluid present on Day 0 was associated with a smaller litter size. This is the first study to establish the prevalence and effect of uterine luminal free fluid in bitches. We postulate that as the uterine luminal free fluid may be detected in normal bitches before mating and has no effect on the pregnancy rate or litter size in either group, this fluid is “physiological.” However, it seems that the later in estrus the uterine fluid is present, the more likely it is to have an adverse effect on fertility. In some cases, this is manifest as a reduced litter size, while, importantly, the presence of fluid 5 to 14 days after ovulation is strongly associated with an absence of pregnancy, both in bitches with and without endometrial hyperplasia. We propose that ultrasonographic detection of uterine luminal free fluid after mating may be a useful prognostic indicator for pregnancy outcome, which may allow the targeting of treatments specifically to a population of animals that presumably develop mating-induced endometritis.