Ultrasonographic Evaluation of Cavum Septum Pellucidum Dimensions and Morphology in the Second Half of Pregnancy: A Prospective Cross-Sectional Study.

OBJECTIVE: To determine normative ultrasonographic dimensions, morphology, and visualization rate of the cavum septum pellucidum (CSP) in structurally normal singleton fetuses between 19 and 40 weeks of gestation and to assess correlations with gestational age and fetal biometric parameters. METHODS: This prospective cross-sectional study enrolled 600 consecutive singleton pregnancies with normal fetal anatomy at Beheshti Hospital, Isfahan, Iran (2024-2025). Transabdominal ultrasound was performed in standard ISUOG-recommended axial planes. CSP visualization status, morphology (rectangular, triangular, oval, square, or circular), length, and width were recorded together with biparietal diameter (BPD) and head circumference (HC). Data were analyzed using descriptive statistics, Pearson's and Spearman's correlations, and chi-squared tests. RESULTS: The CSP was visualized in 552 of 600 fetuses (92.0%). Mean CSP length was 8.27 ± 2.94 mm and mean width was 6.60 ± 2.74 mm, with rectangular morphology predominating (71.7%). Mean length increased modestly from 7.01 ± 2.62 mm (19-23 weeks) to 8.67 ± 2.98 mm (32-35 weeks) then stabilized, whereas width remained relatively stable. Weak positive correlations were observed between CSP length and gestational age (r = 0.132, p = 0.002) and HC (r = 0.156, p < 0.001); however, the small effect sizes indicate limited explanatory and clinical significance. No significant correlations were observed for width or morphology with biometric parameters. Nonvisualization (8.0%) occurred exclusively at or after 19 weeks. CONCLUSION: This study provides contemporary, population-specific normative data for fetal CSP in the second and third trimesters in an Iranian cohort. CSP length shows modest gestation- and HC-dependent growth, while width and morphology remain largely independent of standard biometry.