Fetal head and abdominal circumferences: effect of profile shape on the accuracy of ellipse equations.

Images of 122 fetal heads and abdomens were obtained with ultrasound for the determination of head and abdominal circumferences. The profile shapes were characterized by the cephalic or abdominal index (D1/D2 x 100), D1 and D2 being the shortest and longest diameters of either the head or abdomen. The mean of the cephalic index was 79.1% +/- 5.2%, and that of the abdominal index was 89.1% +/- 7.0%. All equations were equally accurate in calculating head and abdominal circumferences regardless of the profile shape. Therefore, the simplest equation, (D1 + D2) x 1.57, should be utilized when calculating circumferences except in situations of extreme dolichocephaly.