Biological variation in the biparietal diameter and its bearing on clinical ultrasonography.

Interpopulation differences in the term BPD and its relations to BW stress the need for each institution to analyze its own data. Variations in the BPD as a function of BW do not connote differences in the brain: body weight relationship, because the neurocrania of all term fetuses are not homomorphic. Differentiation of neurocranial form accounts for there being no consistent relationship between the BPD and calculated brain weight in interpopulation comparisons. The wide scatter of BPD and BW values in data from a single source challenges the wisdom of judging individual serial BPD readings by mean population values. Because the correlation between BW and the common indices of maternal size (stature, weight, and surface area) is too low (r = 0.27, 0.27, and 0.35, respectively) to provide a useful means of determining the fetal growth potential, each fetus should be used as its own control in serial cephalometry.