The uterine artery resistivity index is affected by embryo resorption rate at canine pregnancy diagnosis

The uterine artery resistivity index (RI) obtained by Doppler ultrasonography has become a common method in evaluating uterine blood flow during pregnancy for assessing canine fetal and placental conditions. During pregnancy, embryonic vesicle resorptions can be observed by ultrasound and have been found to increase with the aging of the dam. The RI is higher during induced abortion in bitches, while its relationship with early embryo resorptions is not yet clearly defined. The aim of this study was to evaluate whether the uterine artery's haemodynamics at pregnancy diagnosis are related to embryo resorption rates and other pregnancy and dam parameters. The ultrasound examinations were performed 25 ± 4 days post-ovulation (between days 21 and 29). During the examinations, the animals were in lateral recumbency, and both uterine horns were scanned to determine the number and viability of embryos and resorption sites. The left uterine artery was visualized, and the RI was determined. The breed, weight, reproductive anamnesis, and age of the animals were recorded. Forty-four bitches were included, weighing 2.8-39.2 kg, for 50 pregnancy diagnoses performed at ages ranging from 18 to 88 months. Bitches were allocated to different size groups according to their body weight and breed: Medium (M, 10-40.0 kg), Small (S, 5-9.9 kg), and Toy (XS, <5 kg). A linear regression was used to model the RI as the outcome, with size group and age of the dam (above or below the median value), litter size (sum of viable embryos and resorptions), gestational age (days post-ovulation), parity (multiparous vs. primiparous), cystic endometrial hyperplasia (present or not), and either the percentage of resorptions (on litter size) or simply their presence as predictors. The overall model was statistically significant (P = 0.029), with an R² index of 0.296. A significant relationship with the response was found for gestational age (P = 0.012) and percentage of resorptions (P = 0.045). No significant relationships were found for the other parameters. As reported in the literature, as gestational age increased, RI decreased. Embryo resorptions were observed in 17/50 (34.0 %) pregnancies. As the percentage of resorption sites within the pregnancy increased, the RI also increased. Conversely, the simple presence or absence of resorption sites had no statistically significant effect on the RI. This study describes, for the first time, a significant impact of embryonic resorptions on uterine artery RI at pregnancy diagnosis. Whether these changes are a cause or effect of the abnormal pregnancy remains to be elucidated.