Deep vein thrombosis (DVT) is the formation of a blood clot in a vein that runs between muscles deep inside your body. Your veins are the blood vessels that return blood to your heart. Though a DVT can occur anywhere in the body, it's most common in a vein in the pelvis, thigh or lower leg.

DVT is a serious problem because the clot can break loose and travel through your blood. It's then called an embolus. If it reaches your lungs, it is known as a pulmonary embolism (PE) and can cause life-threatening lung damage.

You can have a DVT but have no symptoms. If you do have symptoms, they may include:

  • Warmth over the area of the DVT
  • Swelling of the leg or thigh
  • Redness of the skin over the DVT
  • Pain or tenderness, especially when standing or walking

Let your doctor know right away if you have any of these symptoms.

Sometimes, a PE is the first sign of DVT. Symptoms of PE come on suddenly and can include chest pain and difficulty breathing. If you have these symptoms, call 911 or go to the emergency room.

DVT may be caused by an injury to the inside of a vein, slow blood flow through a vein or a condition that increases blood clotting. You're more likely to develop DVT if you've had this condition before or if you have a family history of DVT or PE. Other risk factors include:

  • Inactivity, such as being bedridden or taking long trips
  • Recent surgery or injury
  • Taking hormone replacement therapy or birth control pills
  • Pregnancy, or having recently given birth
  • Having cancer or recent cancer treatment
  • Being older than 60
  • Excessive weight
  • Having a tube inserted into a major vein for a medical treatment (central venous catheter)
  • Smoking
  • Being born with a disorder that increases blood clotting