This paper proposes a method of approximating the fetal calvarial volume (CV) by measuring the vertical calvarial diameter in addition to the biparietal diameter and the fetal long skull diameter. The methods of measurement and calculation are discussed, and graphic analysis is presented. The approximate CV and the sum of the three fetal skull diameters (3D) correlate well with fetal gestational age regardless of individual variations in fetal skull shape and normal molding. Two least squares regressions for estimating fetal ages from the CV and 3D are given. Three cephalic indices in the transverse, sagittal, and coronal planes are also discussed.
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