An abdominal aortic aneurysm (AAA) is a balloon-like enlargement of the aorta, a vessel one-inch (2.5 cm) in diameter that is the largest blood vessel in the body, supplying blood to most organs. An enlarged aorta is considered to be an aneurysm when it expands to 150% of its normal size. On average, an AAA grows about 10% per year or 0.5 cm per year in larger aneurysms (over 6 cms).