The cavum septum pellucidum is a fluid-filled cavity located between the membranes of the septi pellucidi, which should always be visualized sonographically in normal fetuses between 18 and 37 weeks gestation and in approximately 50% of term infants. The cavum vergae, a cavity within the septum pellucidum, is located posterior to an arbitrary vertical plane formed by the columns of the fornix. We present a case in which markedly dilated cavum septum pellucidum et vergae noted at 29 weeks gestation was associated with midgut malrotation and volvulus, and review the literature pertaining to this unusual prenatal ultrasonographic diagnosis.