Log-linear allometry of normal fetal craniofacial growth

Normative data on human craniofacial growth during the fetal period and important to provide a broader perspective on normal morphogenesis as well as to serve as reference for analyzing craniofacial syndromes in which growth has gone awry. Over a 19-year period, the Teratology Unit at the University of Michigan Medical Center has collected data on 2,568 legally donated fetuses that have undergone necropsy examination at various gestational ages. From previous analyses, 609 of the total fetal population (25%) were designated as typical for age or body weight on the basis of normal morphology, absence of maceration, and general growth symmetry. Of the 609 fetuses reviewed, 54 were excluded secondary to incomplete data. The remaining 555 constitute the basis of this study. Seven craniofacial measurements were recorded, including head circumference (HC), brain weight, inner canthal and outer canthal distances, and distances from nasion to menton, outer canthus to tragus and auditory meatus to vertex. Statistical analysis was carried out using the single-factor allometric model of Sewall Wright. Size was estimated as the first unstandardized principal component of the logarithms of lengths and of cube roots of weights, and then allometry was expressed in the regressions of each log variable on size. Significant allometry was found as were significant differences in errors about the allometric relation, but no evidence for more than a single factor or of "nonlinearity" in the regression curves was noted. Although there were differences of specific allometric coefficients between the various measurements (i.e., the slope of the curve for IC was significantly smaller than the slope of the curve generated for HC), these specific growth rates remain in relatively strict proportion to one another from early in gestation (body weight, 54.2 gm) to later in gestation (body weight, 1,000 gm).