Growth in the lateral part of the human skull during the second trimester.

The aim of the present study was to report dimensional changes in the lateral part of the prenatal human skull during the second trimester. The sample comprised 37 fetuses obtained in connection with requested autopsy. Gestational ages (GA) were available for 27 fetuses, ranging from 13 to 24 weeks. Crown-rump lengths (CRL) of 23 fetuses ranged from 78 to 228 mm. For 21 fetuses both CRL and GA were available. After excision of the midsagittal segment of the skulls by two paramedian sections from the frontal bone to the foramen magnum through the optic foramina, an incision was made through the middle of the occipital squama, bisecting the skull. The present study was based on the skull components of the resulting 74 lateral cranial tissue blocks. The tissue blocks were radiographed with a Hewlett Packard Faxitron unit. Seven reference points were digitized from each film and eight linear dimensions were calculated. Method errors ranged from 0.12 mm to 0.52 mm. The dimensions were related to crown-rump length (CRL) and lateral segment developmental stages (LSS). The increases in the linear dimensions of the anterior part of the lateral skull segments were similar to those found previously for the median skull segment. The dimensions of the anterior part of the lateral skull segments were found to increase by 150% during the second trimester, while those of the posterior part increased by 100% during the same period. Between maturity stages LSS 4 and 5 the dimensions increased by 30-50%, whereas less increase was observed between stages 5, 6, 7, and 8. This reduction was attributed to the progressive ossification of the cranial base. The almost horizontal distance from the mandibular condyle (condylion) to the anterior tympanic ring did not change significantly during the second trimester. Comparison of the bilateral skull segments showed no systematic asymmetry, but a significant individual biological variability in symmetry with standard deviations of 0.7-1.8 mm was found.