Association of cephalic index with corpus callosum and cavum septi pellucidi measurements during second-trimester neurosonography

Abstract Objectives This study aimed to investigate whether physiologic variation in cephalic index during routine second-trimester neurosonography influences corpus callosum (CC) and cavum septum pellucidum (CSP) measurements. Methods In this retrospective cross-sectional study, prospectively collected ultrasound data from singleton fetuses at 19–24 weeks of gestation from 2023 to 2025 were analyzed. A total of 1,701 structurally normal fetuses were included and categorized into lower, intermediate, and higher CI groups reflecting physiologic variation in fetal head shape. CC length and CSP width were measured using standardized techniques. Associations between CI and measurements were evaluated using multivariable linear regression adjusted for gestational age. Results CC length did not differ significantly across CI groups (p=0.075), whereas CSP width showed a statistically significant but small difference (p=0.031). Gestational age was strongly associated with both CC length (β=0.842, p<0.001) and CSP width (β=0.448, p<0.001). CI demonstrated a modest independent association with CC (β=0.029, p=0.025) and CSP (β=0.092, p<0.001), with limited clinical relevance. Conclusions Although small associations between CI and midline measurements were identified, gestational age remained the predominant determinant of CC and CSP size. These findings suggest that fetal head-shape variation during the mid-trimester period is unlikely to substantially affect routine clinical interpretation.