Our aims were: (1) to perform an echoanatomic correlation study, in order to confirm that the structure identified as the optic chiasm (OC) on ultrasound (US) is indeed this anatomical structure; (2) to assess and compare the reproducibility of two- (2D) and three-(3D) dimensional US in measurement of the OC in normal fetuses; and (3) to assess whether the spatial orientation of the OC changes with increasing gestational age.