Estimating fetal age: effect of head shape on BPD

Several recent obstetrical sonographic examinations in this department demonstrated that variations in the shape of the fetal skull (e.g., dolichocephaly, brachycephaly) may adversely affect the accuracy of the biparietal diameter (BPD) measurement in estimating fetal age. In each case the cephalic index of the fetal skull (short axis/long axis x 100) was in either the dolichocephalic or brachycephalic range based on established postnatal criteria. Consequently, normal values were determined (mean, 78.3) for the cephalic index in utero based on 316 obstetrical sonographic studies performed at 14--40 weeks. Preliminary experience indicates that a cephalic index greater than 1 SD from the mean (less than 74, greater than 83) may be associated with a significant alteration in the BPD measurement expected for a given gestational age, and that the head circumference can be used effectively as an alternative means of establishing gestational age.
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