Relationship between days before parturition and fetal kidney length, cortical thickness, medullary thickness and their ratio in dogs.

Canine gestational age can be estimated based on fetal kidney length (L) although accuracy, sensitivity and specificity decrease during the last 5 days of pregnancy. In humans, fetal renal cortical and medullary thickness (CT and MT) and their ratio (CT/MT) are described as useful tools for monitoring the development of fetal kidneys. The aim of this study was to evaluate a potential relationship between canine fetal kidney parameters and gestational age. Ten clinically healthy pregnant bitches of different breeds were monitored by ultrasound at least twice from -10 to 0 days before parturition (dbp). L, CT, MT and CT/MT were measured on the three most caudal fetuses of both uterine horns. Statistical analysis was performed using a linear mixed model considering maternal size (small ≤10 kg, N = 4, medium 11-25 kg, N = 3, and large 26-40 kg, N = 3) as fixed effect, dbp (-10 to 0) and litter size as covariates, and the bitch as a random and repeated effect. Dbp and L, CT, MT showed a negative and linear correlation, and their estimated regression coefficients were -0.68 ± 0.14, -0.04 ± 0.01 and -0.12 ± 0.02 mm, respectively (P < 0.01). CT/MT decreased as parturition approached, whereas MT and CT increased. A statistically significant difference was found for L between small and large bitches (17 ± 1 vs 24 ± 2 mm, P = 0.03) and, for CT, between small, medium and large bitches (1.57 ± 0.04 vs 1.77 ± 0.04 vs 1.99 ± 0.05 mm, P < 0.001). None of the renal parameters were affected by litter size. L, CT, MT and CT/MT are related to dbp and may be considered for predicting parturition in dogs.