Two-dimensional cross-sectional method: a new method for observing fetal corpus callosum in the second and third trimesters

Objective To determine the transverse sections of corpus callosum on two-dimensional ultrasound and to establish the reference ranges for fetal corpus callosum dimensions on the transverse sections of normal foetus from 20 weeks to full term. Methods From June 2018 to December 2018, 10 singleton fetuses with no structural abnormalities diagnosed by prenatal ultrasound were selected as study subjects at Shenzhen Maternal and Child Health Hospital, whose parents consented to autopsy at 20-30 weeks of pregnancy due to inevitable abortion for ″cervical incompetence″ or ″unplanned birth″. A series of transverse sections obtained by three-dimensional oblique technique and frozen autopsy specimens were observed, and the characteristics of each section were summarized. The stable sections of corpus callosum on the transverse section were selected. From June 2018 to December 2018, 670 singleton pregnant women underwent routine ultrasound examination at the Ultrasound Department of our hospital. The corpus callosum diameters including the longest diameter of the corpus callosum, left to right genu diameter, anterior to posterior genu diameter, left to right splenium diameter, anterior to posterior splenium diameter, left to right body diameter, and anterior to posterior body diameter, and the angle of genu and splenium were measured at the cross-sectional plane mentioned above. Forty fetuses were randomly sampled and tested for repeatability between the two measurements. The normal reference ranges of the parameters of corpus callosum on the transverse section of the brain of each gestational week were established, and the correlation between the measured data and the scatter plot of gestational week was analyzed in parallel to establish regression equations. Results Three cross sections of corpus callosum obtained by the two different methods showed the same anatomical structures. There was no significant difference in the repeatability test between two observers. The scatter plots showed that the parameters of normal fetal corpus callosum were positively correlated with gestational age (r=0.232, 0.343, 0.284, 0.182, 0.913, 0.895, 0.748, 0.787, 0.736, 0.684, and 0.734, respectively, P<0.01). The regression equations were Y=0.514X+ 52.214, Y=0.785X+ 42.897, Y=0.600X+ 47.327, Y=0.436X+ 53.056, Y=0.117X+ 0.019, Y=0.070X-0.169, Y=0.013X+ 0.054, Y=0.065X+ 0.180, Y=0.014X+ 0.064, Y=0.047X+ 0.547, and Y=0.027X-0.116, respectively. Conclusions The serial transverse sections based on 3D oblique technique and frozen autopsy specimens are relatively stable and can be used to evaluate the structure of corpus callosum. The reference charts for normal fetal corpus callosum of different gestational ages may be useful for prenatal evaluation of congenital corpus callosum anomalies. Key words: Transverse section; Fetus; Corpus callosum; Second and third trimesters; Reference charts