Abdominal aortic aneurysms in neonates and infants are rare and are usually associated with infection, vasculitis, connective tissue disorder, or iatrogenic trauma such as umbilical catheterization. An idiopathic congenital abdominal aortic aneurysm is the least common category and there are few descriptions of the imaging features. We present the antenatal and postnatal imaging findings of an idiopathic congenital abdominal aortic aneurysm including the findings on US, MRI and CT.