Real-time ultrasound imaging for predicting ovine fetal age

Fetal head width and thoracic depth were observed via real-time ultrasound imaging in 12 single-, 12 twin- and 4 triplet-bearing BooroolaxSouth Australian Merino ewes at weekly intervals during mid pregnancy. Fetal age varied by a maximum of 24 h. Relationships between fetal age and the linear measures (head width, thoracic depth) were determined within litter size categories, using both linear and quadratic terms. All relationships tested for linearity were statistically significant (P<0.05). A curvilinear relationship (P<0.05) was observed for fetal age and thoracic depth of twin fetuses. Significant (P<0.05) curvilinear relationships were also observed for fetal age and head width, and fetal age and thoracic depth when litter size data were pooled. We conclude that head width and thoracic depth are acceptable linear measurements for estimating fetal age in the ovine species. Implications of these results for research and for the commercial field are discussed.