When blood does not flow smoothly, there is a chance that the blood can form clots. If a blood clot has formed in the heart, it can be carried by the bloodstream to other areas of the body (thromboembolism). This is dangerous because the clot can become wedged in a smaller blood vessel and decrease the flow of blood to the area that blood vessel supplies.
A blood clot that travels to the small blood vessels in the brain can decrease or block blood flow to the brain, causing injury to the brain (ischemic stroke).
The chance of forming blood clots is different from person to person. Atrial fibrillation is not the only factor that creates blood clots; some people may have other medical problems that can increase the chance of forming blood clots or having a stroke. Your doctor will consider your situation to decide whether you have a low or high risk of blood clots or stroke.