Comparative Foetal Biometry in Ovine and Caprine Pregnancies by Means of Ultrasonography

B-mode ultrasonography is widely used for pregnancy monitoring in small ruminants, yet direct longitudinal comparisons of ovine and caprine foetal biometry under a common protocol remain scarce. This study characterised foetal development and derived biometric predictors of gestational age in 27 primiparous Lacaune ewes (42 foetuses) and 14 primiparous native Greek does (24 foetuses). Fifteen biometric parameters were measured weekly by transabdominal ultrasonography from gestational week 3 until parturition, and population-specific linear equations were derived for each parameter. Thoracic diameter, biparietal diameter and thoracic diameter at the sixth rib gave the most accurate within-sample gestational-age estimates in both populations (R2 = 0.97–0.99; RMSE ≤ 0.9 weeks), with femur length and abdominal diameter also performing well. Foetal size and growth differed between the two populations for most parameters, and the number of offspring per dam and foetal sex were further associated with biometric variation, particularly in does. Late-gestation measurements correlated with birth weight (r ≤ 0.76 in does), and exploratory models explained up to 57% of its variance. These findings provide detailed longitudinal reference data and practical, population-specific equations for estimating gestational age, useful for flock and herd management, although independent validation is required before extrapolating them to other breeds or flocks.