Brain aneurysms, also known as an intracranial aneurysms, develop from the thinning of artery walls. Arteries are blood vessels that carry oxygenated blood away from the heart and off to the rest of the body.  When these weaknesses appear in the blood vessels that supply blood the brain, they can balloon, fill with blood, and in rare cases rupture and cause a stroke.  Brain aneurysms are considered common in the United States, as this country sees 200,000 to 3 million cases per year.