Understand Your Risk for Excessive Blood Clotting

Many factors can lead to excessive blood clotting, leading to limited or blocked blood flow. Blood clots can travel to the arteries or veins in the brain, heart, kidneys, lungs and limbs, which in turn can cause heart attack, stroke, damage to the body's organs or even death.

Acquired Risk Factors

Genetic Risk Factors

The genetic, or inherited, source of excessive blood clotting is less common and is usually due to genetic defects. These defects often occur in the proteins needed for blood clotting and can also occur with the substances that delay or dissolve blood clots.

You're more likely to have a genetic cause of excessive blood clotting if you have:

  • Family members who have had dangerous blood clots
  • A personal history of repeated blood clots before the age of 40
  • A personal history of unexplained miscarriages

Other Risk Factors - Diseases and Conditions

Many diseases and conditions can cause excessive blood clotting, or hypercoagulation. Certain diseases and conditions are more likely to cause clots to form in specific areas of the body.

Conditions that can trigger excessive blood clotting in the heart and brain:

  • Atherosclerosis is a disease in which a waxy substance called plaque builds up inside your arteries. Over time, the plaque may rupture. Platelets clump together to form clots at the site of the damage.
  • Vasculitis is a disorder that causes the body's blood vessels to become inflamed. Platelets may stick to areas where the blood vessels are damaged and form clots.
  • Diabetes increases the risk of plaque buildup in the arteries, which can cause dangerous blood clots. Nearly 80 percent of people who have diabetes will eventually die of clot-related causes.
  • Heart failure is a condition in which the heart is damaged or weakened. When the heart can't pump enough blood to meet the body's needs, blood flow slows which can cause clots to form.
  • Atrial fibrillation, the most common type of arrhythmia, or irregular heartbeat. Atrial fibrillation can cause blood to pool in the upper chambers of the heart and can cause clots to form.
  • Overweight and obesity refer to body weight that's greater than what is considered healthy. These conditions can lead to atherosclerosis, which increases the risk of clots.
  • Metabolic syndrome is the name for a group of risk factors that increases your chance of having heart disease and other health problems, including an increased risk of forming blood clots.

Conditions That Can Trigger Excessive Blood Clotting in the Limbs

  • Deep vein thrombosis (DVT): Blood clots can form in the veins deep in the limbs, a condition called deep vein thrombosis or DVT. DVT usually affects the deep veins of the legs. A blood clot in a deep vein can break off and travel through the bloodstream. If the clot travels to the lungs and blocks blood flow, the condition is called pulmonary embolism.
  • Peripheral artery disease (PAD): A narrowing of the peripheral arteries, most commonly in the arteries of the pelvis and legs. PAD is similar to coronary artery disease (CAD) and carotid artery disease. All three of these conditions are caused by narrowed and blocked arteries in various critical regions of the body. Hardened arteries (or atherosclerosis) in the coronary artery region, restricts the blood supply to the heart muscle (View an illustration of coronary arteries). Carotid artery disease refers to atherosclerosis in the arteries that supply blood to the brain.
  • Atherosclerosis: A disease in which plaque builds up in the wall of an artery. PAD is usually caused by atherosclerosis in the peripheral arteries (or outer regions away from the heart). Plaque is made up of deposits of fats, cholesterol and other substances. Plaque formations can grow large enough to significantly reduce the blood's flow through an artery. When a plaque formation becomes brittle or inflamed, it may rupture, triggering a blood clot to form. A clot may either further narrow the artery, or completely block it.

Medicines

Medicines can disrupt the body's normal blood clotting process. Medicines containing the female hormone estrogen are linked to an increased risk of blood clots. Examples of medicines that may contain estrogen include birth control pills and hormone therapy.

Other Factors

Many other factors can lead to excessive blood clotting:

  • Smoking raises the risk of unwanted blood clots and makes it more likely that platelets will stick together. Smoking also damages the lining of the blood vessels, which can cause clots to form.
  • Increased homocysteine levels, linked to a high risk of vascular disease. Increased levels of this substance may damage the inner lining of the arteries.
  • Pregnancy. Women are more likely to develop blood clots when they're pregnant due to an increase in platelets and clotting factors. The uterus can also compresses the veins slowing blood flow, which can lead to blood clots.
  • Prolonged bed rest. Several days or weeks in bed from surgery or illness can increase risk of excessive blood clotting.
  • Use of birth control pills or hormone replacement therapy. These can slow blood flow and cause clotting.
  • Cancer. Some types of cancer increase the proteins that clot your blood.
  • HIV and HIV treatments. The risk of blood clots is highest in HIV patients who have infections, are taking certain medicines, have been hospitalized, or are older than 45.
  • Dehydration, a condition in which your body doesn't have enough fluids. This condition causes blood vessels to narrow and blood to thicken, raising risk for blood clots.
  • Organ transplants and implanted devices, such as central venous catheters and dialysis shunts. Surgery or procedures done on blood vessels may injure the vessel walls. This can cause blood clots to form. Also, catheters and shunts have a man-made surface that may trigger blood clotting. 

Written by American Heart Association editorial staff and reviewed by science and medicine advisers. See our editorial policies and staff.