- Coarctation of the aorta is characterized by narrowing of the distal aortic arch. This obstructive lesion may reduce the blood flow in the fetal aortic arch, leading to arch hypoplasia, although in some cases this may only be clinically evident after birth. A sonographic finding described in neonates with coarctation of the aorta is the "contraductal shelf," which has been proposed to represent residual fibrous tissue derived from the ductus arteriosus. B-flow is a display modality in 4-dimensional (4D) sonography that enhances signals from weak blood reflectors from vessels, suppresses strong signals from surrounding tissues, and is angle independent. Here, we report a case of coarctation of the aorta diagnosed in utero in which B-flow imaging provided important insight into the location and nature of this aortic arch anomaly.