LOG-LINEAR ALLOMETRY OF NORMAL FETAL CRANIOFACIAL GROWTH

Normative data on human craniofacial growth during the fetal period an d important to provide a broader perspective on normal morphogenesis a s well as to serve as reference for analyzing craniofacial syndromes i n which growth has gone awry. Over a 19-year period, the Teratology Un it 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 wei ght on the basis of normal morphology, absence of maceration, and gene ral growth symmetry. Of the 609 fetuses reviewed, 54 were excluded sec ondary to incomplete data. The remaining 555 constitute the basis of t his study. Seven craniofacial measurements were recorded, including he ad circumference (HC), brain weight, inner canthal and outer canthal d istances, and distances from nasion to menton, outer canthus to tragus and auditory meatus to vertex. Statistical analysis was carried out u sing the single-factor allometric model of Sewall Wright. Size was est imated as the first unstandardized principal component of the logarith ms of lengths and of cube roots of weights, and then allometry was exp ressed in the regressions of each log variable on size. Significant al lometry 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 vari ous measurements (i.e., the slope of the curve for IC was significantl y smaller than the slope of the curve generated for HC), these specifi c growth rates remain in relatively strict proportion to one another f rom early in gestation (body weight, 54.2 gm) to later in gestation (b ody weight 1,000 gm).