Thinned Cavum Septum Pellucidum Leaflets Mimicking Septal Agenesis on Sonography

Abstract Background: Visualization of the cavum septum pellucidum (CSP) during the second and third trimesters is an important marker of normal central forebrain development. Nonvisualization of the CSP on prenatal ultrasonography raises concern for septal agenesis and associated neurodevelopmental abnormalities. However, markedly thinned CSP leaflets may mimic true agenesis. Objective: We aimed to evaluate septal leaflet thickness in fetuses referred with suspected septal agenesis on prenatal ultrasonography, in whom the CSP was visualized on fetal magnetic resonance imaging (MRI), and to differentiate true septal agenesis from thinned CSP leaflets. Materials and Methods: This retrospective observational study included 18 fetuses referred for fetal MRI between May 2020 and December 2025 for suspected nonvisualization of the CSP on prenatal ultrasonography. The presence or absence of CSP leaflets, leaflet thickness, and associated central nervous system anomalies were assessed. A gestational age-matched control group of 10 fetuses with normal fetal brain MRI was included for comparison. Results: True septal agenesis was confirmed in 10 of 18 fetuses. Eight fetuses demonstrated thinned but identifiable CSP leaflets. The mean total septal leaflet thickness in the thinned CSP group was 1.7 mm compared with 3.27 mm in the control group. No major structural anomalies were identified in this group. Postnatal follow-up revealed unremarkable early clinical outcomes. Conclusion: Thinned CSP leaflets are an important cause of apparent nonvisualization of the CSP on prenatal ultrasonography. Fetal MRI reliably differentiates this benign developmental variant from true septal agenesis and plays a crucial role in accurate prenatal diagnosis and counseling.