Fetal renal ultrasonography in canine pregnancy: relationship with maternal and fetal metrics for assessing fetal maturity

Abstract

Accurate estimation of gestational age is essential in canine obstetrics to optimize the timing of parturition and reduce neonatal risk. This study investigated the correlation between fetal kidney biometry and established gestational indicators in pregnant bitches. Fifty clinically healthy bitches underwent serial ultrasonographic evaluations between Days 40 and 63 of gestation. A total of 147 fetal kidney measurements were recorded, including longitudinal length, vertical width, and cortex-to-medulla (C/M) ratio. Biparietal diameter (BPD), maternal serum progesterone concentration, body weight, and litter size were also assessed. Fetal kidney area showed a strong positive correlation with gestational age (r = 0.64, p < 0.001), BPD (r = 0.80, p < 0.001), and maternal body weight (r = 0.43, p < 0.001), and a moderate negative correlation with maternal progesterone levels (r = -0.47, p < 0.001). A receiver operating characteristic (ROC) curve was constructed to evaluate the diagnostic performance of fetal kidney area in discriminating between fetal categories defined by biparietal diameter (BPD). The analysis identified an optimal cut-off value of 6.78 cm2, achieving a sensitivity of 100 % and a specificity of 65.3 %. Linear regression revealed a predictive model between renal area and BPD (R2 = 0.623; p < 0.001). The C/M ratio did not significantly correlate with gestational age or maternal factors. These findings support fetal kidney area as a reliable ultrasonographic parameter for estimating gestational age in bitches, particularly in late pregnancy. Its application may complement traditional biometric markers and improve clinical decision-making regarding optimal timing of parturition.