Objective      To analyze the value of the MRI and ultrasonography（US）in the prenatal diagnosis of absence of cavum septum pellucidum（CSP）.       Methods      Fetal absence of CSP was suspected in 67 pregnant women by prenatal US from January 2009 to June 2011 in Qingdao Women Children Hospital. Prenatal systemic ultrasonography was performed. The findings of prenatal US and MR images were analyzed. Diagnostic accuracy was compared in these 2 imaging studies through the pathological and image data obtained after postpartum/odinopoeia

       Results      Among the 67 fetuses

there were 31 (46.3%) with agenesis of corpus callosum (ACC)

22 (32.8%) with holoprosencephaly

and 14 suspected to be with isolated absence of CSP. Only 3 fetuses were full-term born

and identified without any obvious congenital abnormality in nervous system. The oldest was 2 years and 1 month old. And they belonged to isolated absence of CSP. According to the pathological and image data after postpartum/odinopoeia

MRI had 100% accuracy in the diagnosis of prenatal CSP

100% for ACC

93.3% for holoprosencephaly and 75% for isolated absence of CSP.       Conclusion      When the absence of CSP is found in US diagnosis

further examination should be performed continuously and analyzed with gestational weeks. And attention should be paid to its correlation with malformations in central nervous system. For isolated absence of CSP

malformation must be carefully diagnosed. MRI is more accurate in diagnosis for absence of CSP than US．