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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 can cause heart attack, , organ damage or even death.

Acquired risk factors

• Overweight and obesity
• Advanced age
• Pregnancy
• Prolonged bed rest due to surgery, hospitalization or illness
• Long periods of sitting, such as car or plane trips
• Use of birth control pills or hormone replacement therapy
• Cancer
• Smoking
• Diabetes
• Inflammation
• Antiphospholipid syndrome (an autoimmune disorder)
• Dehydration may cause blood vessels to narrow and blood to thicken, raising the risk of blood clots.

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 are 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 age 40.
• A personal history of unexplained miscarriages.

Other risk factors – diseases and conditions

Many diseases and conditions can cause excessive blood clotting. 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:

• 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.
• 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 is 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 inflammation and damage the lining of the blood vessels.
• 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 include:

• Deep vein thrombosis (DVT) is a condition where blood clots form in the veins deep in the limbs. 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) is a narrowing of the peripheral arteries, most commonly in the arteries of the pelvis and legs. PAD is similar to coronary artery disease  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: is a disease in which plaque (fat, cholesterol and other deposits) builds up in the wall of an artery. Plaque formations can grow large enough to greatly reduce the blood flow through an artery. When a plaque formation becomes brittle or inflamed, it may rupture, causing a blood clot to form. A clot may further narrow the artery or completely block it.