Comparison of two‐dimensional (2D) and three‐dimensional (3D) ultrasonography for gestational ageing in the early to mid‐pregnant bitch

Accuracy of prediction of the day of parturition in the bitch decreases with advancing pregnancy. We hypothesized that three‐dimensional (3D) volume ultrasonography may be superior for gestational ageing than 2D measurements. Thirty‐two pregnancy examinations were performed in 25 bitches of different breeds 21–34 days after the first mating. Ovulation date from serum progesterone concentrations was estimated for 15 bitches, and parturition date was recorded for all dogs. Inner chorionic cavity (ICC) diameters and ICC length of the conceptus were measured by 2D B‐mode, and ICC volume by 3D ultrasonography (Voluson(®) i, GE Healthcare). Linear or exponential regressions between ICC measurements and time, i.e. days after estimated ovulation or days before parturition, were performed with significance set at p < .05. ICC volume increased exponentially over time, while ICC diameter showed a linear growth. ICC volume had similar accuracy (R (2) = .718, p < .001) to ICC diameter and ICC length (R (2) = .707 and R (2) = .728, p < .001) when analysed in relation to days after estimated ovulation or in relation to days before parturition (R (2) = .818, R (2) = .800 and R (2) = .810, p < .001 respectively). There were strong (R (2) ≥ .875) significant associations between all ICC measurements. In conclusion, ICC volume did not improve canine gestational ageing accuracy, which supports the continued use of the ICC diameter, particularly in light of the difficulty in obtaining 3D measurements. On the basis of ICC volume, the growth of the canine conceptus appeared to be exponential already during the first half of pregnancy.