Describe the anatomical variants and abnormalities in the corpus callosum founded in fetuses with increasing size of the cavum the septum pellucidum and describe a new sing for prenatal screening of abnormalities in the fetal brain. We analyse our database with looking for the diagnosis of increasing size of the CSP between March 2003 to February 2015. The diagnosis of increasing size of the CSP was made in axial and coronal planes. The transducer was placed slightly cephalad so that the septa pellucida could be seen as two short parallel lines in the inner surface of the lateral ventricles. The largest distance of the inner surface of the septa pellucida was measured. Fetuses with diagnosis of increasing size of CSP were identified and reviewed. The fetal neurosonography and the anatomical characteristics were divided into the shape of the corpus callosum, abnormal segments or patterns of disgenesis. 26 cases were diagnosed with increased size of CSP. In 19/26, 73% of the cases were sings of callosal disgenesis. In 3 cases we found other abnormalities like cyst of CSI and CMV infection. 2 cases were normal fetal brain and the diagnosis was anatomical variant. In a significant proportion of cases with prenatal finding of increased size of CSP the shape of the CSP is abnormal and should be the first sign of other abnormalities of fetal brain. This sign should be considered an additional indirect sign of fetal brain abnormalities.