What Are the Causes of Blood Clots in the Legs?

Blood clots occur when the blood hardens from a liquid to a solid. A blood clot that forms inside a blood vessel and remains there is called a thrombus. A deep vein thrombosis, or DVT, usually occurs in the leg veins. Many factors can increase the risk of developing blood clots in the legs, especially if several risk factors occur at the same time.

Is This an Emergency?

If you are experiencing serious medical symptoms, seek emergency treatment immediately.

Vein Damage

According to the National Heart Lung and Blood Institute, a blood clot may form in the legs if damage occurs to the vein's inner lining. The damage may result from surgery, serious injury, inflammation or an immune response. Injury or surgery to the veins can slow blood flow, increasing the risk of blood clots.

General anesthetics used during surgery can widen the veins, which increases the risk of blood pooling and then clotting, reports MayoClinic.com.

Slow Blood Flow

Blood clots can form in the legs due to a lack of motion. Inactivity for a long period of time can cause a sluggish or slow blood flow, notes the National Heart Lung and Blood Institute. Prolonged sitting, paralysis and recovering in bed after surgery cause the legs to remain still. The blood doesn't circulate in the legs because the calf muscles aren't contracting. Lack of circulation causes blood clots to develop, reports MayoClinic.com.


A pregnancy, or recently giving birth can cause blood clots to form in the legs, reports FamilyDoctor.org. Pregnancy increases the pressure in the veins of the pelvis and legs. The risk of blood clots from pregnancy can continue for up to six weeks after delivering a baby, reports MayoClinic.com.

Heart Failure

People with heart failure may develop blood clots in the legs because a damaged heart doesn't pump blood as effectively as a normal heart does. The Mayo Clinic advises that inadequate blood flow to the legs increases the chance that blood will pool and clot.


According to the Mayo Clinic, some forms of cancer can increase the amount of substances in the blood that cause the blood to clot. Cancer treatment increases the risk of blood clots as well.

Inherited Conditions

Certain inherited conditions cause the blood to become thicker than normal, increasing the blood's tendency to clot, states the National Heart Lung and Blood Institute. MayoClinic.com reports that some inherited conditions may not cause problems unless combined with one or more other risk factors.

Other Risk Factors

Other risk factors that cause the blood to clot include taking birth control pills or hormone replacement; a family history of blood clots; smoking; and being overweight or obese, notes the Mayo Clinic.