Midsagittal dimensions of the prenatal human cranium.

The purpose of the present study was to analyze the linear dimensions of the cranial base and the anterior facial heights in the median plane of human fetal crania during the second trimester. The distances measured were related to gestational age (GA), crown-rump length (CRL) and maturation stages of the cranial base (MSS). The material comprised midsagittal tissue blocks of the crania from 52 normal human fetuses aged 13 to 22 weeks GA with CRL from 78 to 230 mm and cranial base maturation from MSS 3 to MSS 7. The measurements of the cranial and facial dimensions were performed on radiographs of these tissue blocks. The study confirmed previous observations regarding the dimensional increase in the linear dimensions of the cranial base and the upper facial height in the second trimester. The assessment of absolute changes was further supplemented by an analysis of the percentage changes. This analysis showed that the percentage changes from MSS 3 to MSS 7 of the linear dimensions of the cranial base and the upper facial heights were similar. On the other hand, the percentage increase in the lower anterior facial height during the second trimester was found to be much larger than that of the cranial base and the upper anterior facial height. It is suggested that this is related to the marked increase of the tooth germs during the period and the concurrent growth of the alveolar processes. Standards for normal prenatal cranial dimensions in relation to stages of maturation in the midsagittal cranial base were provided. These data may be of value for use in prenatal diagnostics.