The ‘early CSP ’ is not a CSP : 3D transvaginal neurosonographic study of fetal cavum septi pellucidi formation and septal development

Abstract

ABSTRACT Objectives To describe the in‐vivo development of the fetal cavum septi pellucidi (CSP) using transvaginal ultrasonography (TVS) and to propose a hypothesis underlying non‐visualization of the septum pellucidum (NVSP). Methods In this retrospective study, three‐dimensional ultrasound volumes of the fetal brain acquired using TVS between 17 + 0 and 24 + 6 weeks' gestation in structurally normal fetuses between January 2022 and November 2025 were analyzed. Multiplanar reconstruction was performed to allow assessment of midline structures. Corpus callosal (CC) length and CSP width were measured. CSP fluid content was scored from 0 to 2 as follows: 0, no fluid visualized within the future CSP; 1, partially fluid‐filled CSP; and 2, completely fluid‐filled CSP. Among fetuses with a CSP fluid score of 2, we further assessed the presence of echogenic strands within the CSP. Associations between gestational age, CC length and CSP parameters were analyzed using linear regression, logistic regression and receiver‐operating‐characteristic (ROC)‐curve analyses. Inter‐ and intraobserver reproducibility were assessed. Results A total of 116 fetuses were included. CSP fluid had an estimated 50% probability of being visualized at 19.03 (95% CI, 18.32–19.75) weeks, with both gestational age (area under the ROC curve (AUC), 0.967 (95% CI, 0.931–1.000)) and CC length (AUC, 0.972 (95% CI, 0.943–1.000)) demonstrating excellent predictive performance for CSP fluid visualization. An optimal threshold for transition into a visualizable CSP structure was seen at a CC length of 11.25 mm. CSP width was associated independently with CC length ( β = 0.19; P < 0.001), but not with gestational age. Prior to cavitation, the region beneath the CC appeared as a non‐fluid layer of intermediate echogenicity, corresponding to the midline glial guidepost apparatus. A triangular CSP configuration was observed in 18.1% of cases with a visible CSP. Reproducibility was excellent, with an intraclass correlation coefficient of 0.980 (95% CI, 0.947–0.993) for CSP width and a weighted Cohen's κ of 0.75 for the ordinal CSP fluid score between observers. Conclusions The fluid‐filled CSP forms gradually, becoming visible at approximately 18–19 weeks after a transient cellular phase beneath the developing CC. CSP appearance is closely linked to CC elongation. We propose a hypothesis in which NVSP, in the presence of a normally formed CC, may result from excessive or disorganized resorption of midline glial guidepost structures and septal leaflets within a critical developmental window. © 2026 International Society of Ultrasound in Obstetrics and Gynecology.