Prediction of parturition date based on ultrasonographic parameters during the last ten days of pregnancy in the canine species

In the canine species, different maternal and fetal parameters are described in literature and may be used in clinical practice to predict parturition date. However, the prediction of parturition date during the last week of pregnancy is still challenging. The aim of this thesis was to investigate parameters indicating fetal development and fetal maturity during the last ten days of canine pregnancy. Fetal gastrointestinal motility (FGM) and fetal kidney parameters (fetometric and echogenicity measurements) were investigated. Private owned healthy pregnant bitches were monitored by ultrasound (US) from one to three times during the last ten days before parturition (dbp). US images of the three most caudal fetuses were obtained and specific parameters (FGM, fetal kidney length, cortical and medullary thickness as well as echogenicity parameters) were measured and analysed using specific dedicated software. A marked increase in fetuses showing FGM (median value from 33% to 100%, P <0.0001) was observed in the last 5 days compared to the previous 5 dbp. The observation of FGM was affected by maternal size, with easier FGM visualization in small size compared to large size bitches. A weak negative correlation was also found between vaginal temperature and FGM. Fetal kidney length (L), cortical (CT) and medullary thickness (MT) increased (estimated regression coefficients -0.68 ±0.14, -0.04 ±0.01 and -0.12 ±0.02 mm; P<0.01), whereas cortico/medullary thickness ratio decreased as parturition approached (0.01 ±0.004, P=0.003). L and CT were also influenced by maternal size. We investigated the application of fetal kidney L formula calculated by Gil et al. (2018) to our data, obtaining a low accuracy of the formula in the range -11/0 dbp (32% ±2 days). Moreover, accuracy was affected by maternal size, showing lower values in large compared to small size bitches (10% ±2 days vs. 60% ±2 days, P=0.007). In the evaluation of renal echogenicity, homogeneity parameters of renal cortex and cortico/medullary ratio decreased as parturition approached (estimated regression coefficients 0.23 ±0.06, P<0.001 and 0.5 ±0.02, P=0.038), being not affected by maternal and litter size during the last 10 days of pregnancy. FGM may be useful in the prediction of parturition date when combined with other maternal and fetal parameters as well as in deciding the best day to start prepartum progesterone monitoring. Further studies are needed before using canine fetal kidney parameters in clinical practice.