Asymmetry of the human skull during fetal growth.

Asymmetry of the human skull in the frontal and basal views has been described previously as a normal feature in adults. However, no investigation has been performed during the fetal period to see when asymmetry first develops. Previous quantitative methods for measuring asymmetry have relied upon defining anatomical points and using geometrical constructions. In the present study image outlines and their centroids (centers of area) were related to each other via a rectangular coordinate reference grid using the technique of morphanalysis. The centroids were shifted to the left relative to the coordinate reference grid indicating a left side expansion. Also the signed area under the cyclical curve, which is zero for a perfectly symmetrical shape, showed a negative shift indicating asymmetry. The cranium was found to be the major site of asymmetry caused by unequal development of the cerebral hemispheres. The asymmetrical development of the temporal lobes caused a greater displacement of the nasomaxillary segment on the left leading to a rotation of the face on the cranium.