Embryonic resorption rates at canine pregnancy diagnoses: A retrospective evaluation

Pregnancy diagnosis in canines is generally performed during embryonic phase, between 19 and 35 days of gestation. At this stage embryonic resorptions can be observed, which, according to the literature, affects 11-26% of conceptuses and 5-43% of pregnancies. Resorption has been hypothesized as a physiological event in uterine overcrowding, however other factors may be involved, such as infectious or non-infectious diseases. This study aimed to retrospectively evaluate the incidence of embryo resorption at ultrasonographic pregnancy diagnosis in different dog breeds, and to identify the main factors determining the occurrence of the resorption sites. 95 pregnancy diagnoses were performed 21-30 days post-ovulation by ultrasound examination on 74 different animals. Breed, weight, and age of the bitches were recorded, and the reproductive anamnesis was collected from their medical records. The overall pregnancy rate was 91.6%. In 48.3% of pregnancies (42/87), at least one resorption site was visible, and embryonic resorption rate was 14.2% (61 resorption sites/431 total structures). Binary logistic regression showed a significant effect of age (P < 0.001), but not the size of the litter (P = 0.357), nor the size of the mother (P = 0.281) or any previous reproductive problems (P = 0.077). Age was significantly higher in pregnancies with resorptions than in normal ones (60.88 ± 18.24 and 40.27 ± 15.74 months, respectively, P < 0.001). The embryonic resorption rate was in line with previous findings, while the incidence of affected pregnancies was higher. Although resorptions may occur physiologically in pregnancies with large litters, a relationship between embryo resorption and litter size was not identified in our sample group, while aging increased the resorption rates. This, together with the occurrence of repeated embryonic resorptions in some bitches included in the study, suggests how resorptions could also be the result of pathological events. The underlying mechanisms and other factors that may be involved need further clarification.