Ultrasonographic assessment of growth and estimation of birthweight in late gestation fetal sheep

Our aim was to identify which ultrasound parameters can be most accurately measured and best predict ovine fetal weight in late gestation. Singleton pregnancies were established using embryo transfer in 32 adolescent ewes, which were subsequently overnourished to produce fetuses of variable size (1720–6260 g). Ultrasound measurements at 126–133 days gestation were compared with fetal weight/biometry at late-gestation necropsy (n = 19) or term delivery (n = 13). Abdominal circumference (AC) and renal volume (RV) correlated best with physical measurements (r = 0.78–0.83) and necropsy/birth weight (r = 0.79–0.84). Combination of AC + RV produced an estimated fetal weight equation [Log EFW = 2.115 + 0.003 AC + 0.12 RV – 0.005 RV²] with highest adjusted R² (0.72) and lowest mean absolute/percentage prediction error (396–550 g/11.1%–13.2%). In conclusion, AC and RV are parameters of choice for assessment of late-gestation ovine fetal growth and can be used to estimate fetal weight with similar accuracy to human fetuses.