An aneurysm is an abnormal widening or ballooning of a portion of an artery, related to weakness in the wall of the blood vessel. Some common locations for aneurysms include:

• Aorta (the major artery from the heart; see abdominal aortic aneurysm and thoracic aortic aneurysm)
• Brain (cerebral aneurysm)
• Leg (Popliteal artery aneurysm)
• Intestine (Mesenteric artery aneurysm)
• Splenic artery aneurysm

Aneurysms are either congenital (present before birth) or acquired. It is not clear exactly what causes aneurysms. Defects in some of the components of the artery wall may be responsible. In certain cases (abdominal aortic aneurysms), high blood pressure is thought to be a contributing factor.

Atherosclerotic disease (cholesterol buildup in arteries) may contribute to the formation of certain types of aneurysms. Pregnancy is often associated with the formation and rupture of aneurysms of the splenic artery (an artery leading to the spleen).