Original sentence: AAAs are commonly divided according to their size and symptomatology.An aneurysm is usually considered to be present if the measured outer aortic diameter is over 3 cm (normal diameter of  aorta is around 2 cm).Aneurysms less than 5.5 cm are therefore usually kept under surveillance until such time as they become large enough to warrant repair, or develop symptoms.For aneurysms under 5.5 cm, the risk of rupture is low, so that the risks of surgery usually outweigh the risk of rupture.If the outer diameter exceeds 5.5 cm, the aneurysm is considered to be large.Ruptured AAA is a clinical diagnosis: the presence of the triad of abdominal pain, shock and pulsatile abdominal mass makes the diagnosis; no further investigations are required for diagnostic purposes, and imaging should not delay surgery.The natural history is of increasing diameter over time, followed eventually by the development of symptoms (usually rupture).The operative mortality has slowly decreased over several decades but remains higher than 40%