Background: The cavum septum pellucidum (CSP) is key midline brain structure routinely assessed in fetal neurosonography. Although CSP visualization is essential for detecting anomalies of forebrain development, normative data for CSP width across gestation remain limited, particularly in South Asian populations. Objective: To evaluate visualization rate of CSP in normal fetuses during second and third trimesters, measure CSP width using transabdominal ultrasound and determine its correlation with gestational age. Methods: A hospital-based descriptive cross-sectional study was conducted among 144 singleton pregnancies between 14–41 weeks at BPKIHS. Transabdominal ultrasonography was performed using standard axial plane at the level of thalami. CSP visualization and CSP width were recorded. Descriptive statistics, chi-square tests and Pearson correlation were applied. Results: CSP was visualized in 130 of 144 fetuses (90.3%). Visualization was optimal between 18–37 weeks, accounting for 84.6% of all visualized cases, whereas visualization before 18 weeks was limited (33.3%). CSP width ranged from 1.81 to 6.8 mm, with mean of 4.23 ± 1.12 mm. CSP width was significantly greater in third trimester than second (4.57 ± 0.95 mm vs. 3.39 ± 1.11 mm; p < 0.001). Gestational age showed a significant positive correlation with CSP width (r = 0.520, p < 0.001). Conclusion: CSP is reliably visualized between 18–37 weeks of gestation. CSP width increases significantly with advancing gestation and act as consistent neurosonographic marker of normal midline brain development. This study establishes normative CSP measurements for a Nepalese population and supports routine CSP assessment during mid-trimester ultrasound.