We examined a fetus at 38 weeks of gestation because of marked disproportion in size of the left and right ventricles. A membrane was detected at the distal end of the ductus arteriosus with significant flow gradient through it. New echocardiographic technologies such as high frame rate imaging and dynamic beam focusing increase spatial and temporal resolution and enhance more precise anatomical diagnosis in the fetus. This ductal membrane might have been related to the transient tachypnea from which the baby suffered after birth.