Fetal head and abdominal circumferences: ellipse calculations versus planimetry

One hundred twenty-two images of fetal heads and abdomens were obtained by ultrasound. The head and abdominal circumferences and their ratios were determined by planimetry and by the ellipse method utilizing four different equations. The accuracy of these equations was assessed by regressing the circumferences and ratios obtained by the equations against those measured by planimetry. All equations were found to be equally accurate in calculating head and abdominal circumferences and the head-to-abdomen circumference ratio. Compared to planimetry, the ellipse method is more accurate in calculating circumferences than when calculating their ratio.