Ductus arteriosus aneurysm (DAA) is characterized by a saccular or fusiform dilatation of the ductus arteriosus. The formation of a thrombus at that level is infrequent and potentially lethal. We present a case of a fetus with a DAA diagnosed during fetal life by 2-D, 3-D, and 4-D STIC echocardiography. Although a thrombus in the ductus and hypertrophy of the right ventricle was identified in postnatal echocardiography, a conservative approach was adopted with good outcome. The features of our case highlight the importance of serial ultrasound in the fetal and neonatal period to assess fetal aneurysm growth as well as to notice any modification of the flow within.