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To assess the sonographic visualization of the fetal optic chiasm (OC) and optic tracts (OTs), establish mid-trimester reference values, and assess the relationship between OC and OT dimensions and cavum septum pellucidum (CSP) measurements.This prospective cross-sectional study included 154 morphologically normal fetuses between 19 and 23 weeks of gestation. The diameters of the fetal OC and OTs were measured using two-dimensional transabdominal ultrasound in a transventricular axial section angled approximately 20–40° caudally. CSP length as well as anterior, middle, and posterior widths were measured by placing the cursor on the inner surfaces in the transventricular section. The average CSP width was included in the analysis.The OC and OTs were successfully visualized in 109 fetuses and included in the analysis. Visualization success in transabdominal axial sections at 19–23 weeks was 70.8 %, with high intraobserver and interobserver reproducibility. Additionally, visualization rates were higher in advanced gestational weeks and in fetuses with breech presentation (p=0.007 and p=0.017, respectively). OC and OT dimensions were positively correlated with CSP length and width, biparietal diameter (BPD), head circumference (HC), and gestational age (p<0.05, for all).The OC and OTs were successfully visualized transabdominally in axial sections with high intraobserver and interobserver reproducibility. Additionally, OC and OT dimensions were positively correlated with gestational age, as well as with BPD, HC, and CSP measurements. [ABSTRACT FROM AUTHOR] more...