Ontogenetic allometry of ossified fetal limb bones

The skeletal components of fetal limbs change in both size and shape throughout gestation. Relative growth of different bones, as well as differences between homologous bones of the upper and lower limbs, are not well known for all stages of fetal development. This study used human fetal skeletal material (N=57) ranging in age from 19 to 40 weeks gestation, and in body mass from 290g to 4650g. Measurements of maximal length and minimal width were taken from the six major long bones: femur, tibia, fibula, humerus, radius, and ulna. These data were log transformed, and growth rates determined from least squares regression of bone length or bone width on body mass and on crown-to-rump length. The results indicated that growth rates are equivalent among bones within a limb, whereas homologous bones in the upper and lower limb grew at different rates. In general, the upper limb bones display negative allometry and the lower limb bones display isometric growth in relation to body mass and crown-to-rump length. Further, there was no difference between growth rates of length and width relative to body mass. The negative slopes of upper-to-lower limb bones in relation to mass confirm the conclusion that lower limb bones grow faster than the upper limb bones from 19 weeks gestation to birth. These results, together with results from similar studies of other periods of fetal development, provide a unified picture of the prenatal growth in human skeletal limbs.