Heart and Blood Conditions


What is thrombosis?

Thrombosis is the process of forming a blood clot in a blood vessel. Most of the time, clotting is a normal, healthy process that stops bleeding.

Blood contains cell fragments called platelets. Platelets stick to wounds in our blood vessels and to each other to help form blood clots, or plugs, where damage occurs.

When a blood vessel is injured, it activates a chain reaction of clotting proteins. This creates a protein thread called fibrin that knits together with the platelets to form a clot.

Types of Blood Clots

Blood clots can form when we damage the surface of our bodies. You may see this in the form of a scab.

Clots can form internally, too. For example, when tissue inside our bodies gets damaged, we may bleed and clot inside. Normally our bodies dissolve these clots over time, once we no longer need them.

Sometimes clotting in blood vessels can be harmful. Health problems that relate to clots include thrombophilia, deep vein thrombosis and peripheral arterial disease.


People with these diseases tend to form blood clots easily, or to form clots when or where they aren't needed.

Deep vein thrombosis (DVT)

This refers to a clot in a vein deep in the body, most often in the lower leg. Lack of movement is one reason for DVT. For example, people who spend many days lying in bed because of illness or surgery may get a clot in one of their legs.

Peripheral arterial disease

Peripheral arterial disease is a reduction of blood flow in the small arteries, most often in the legs. This disease is more common in older people who have some other health problem that makes their arteries narrow over time. A clot can be another reason for reduced blood flow here.

Results of Blood Clots

Any time a clot forms inside the body, there is some risk that it may be so big it blocks blood flow through a blood vessel.

Also, there is some risk that the clot may break free from the place where it formed. Then it will flow through the blood vessels until it reaches a vessel that's too narrow to pass through. There the clot can get lodged and block blood flow.

Blockage of blood flow can be serious, even deadly. Here are some problems that relate to clots that block a vessel:

Pulmonary embolism

Pulmonary embolism is blockage of a blood vessel in the lungs when a clot travels to the lungs from elsewhere.


One reason for a stroke is a clot that blocks a vessel to the brain.

Heart attack

One reason for a heart attack is a clot that blocks a vessel to the heart.

Who Gets Thrombosis?

A blood clot can form in a blood vessel any time there's bleeding inside the body. Risk may be higher after heavy bleeding, such as after a major injury or surgery, because the clotting system may be especially active at that time.

Some other health matters or lifestyle choices may increase risk, too, mainly in adults. For example, cigarette smoking and obesity increase the risk.

Blood clots in pregnancy

Women are at greater risk for blood clots when they are pregnant because the weight of the baby presses on veins in the belly. This reduces blood flow from the legs through the belly back to the heart. Also, the levels of many clotting factors are naturally higher in pregnant women.


These health problems may increase risk of the thrombosis disease thrombophilia:

  • Problems with genes for the substances in blood and platelets that control clotting. These include Factor V Leiden, prothrombin 20210A mutation, proteins C and S and antithrombin. A child with one of these gene problems would have been born with it.
  • Certain problems that happen after birth in people who have lupus or some other disease.

Deep vein thrombosis

People who do not move their legs much, such as people who must stay in bed, have greater risk for deep vein thrombosis (DVT). Long airplane rides and car rides also pose a risk of DVT because sitting decreases blood flow from the legs somewhat.

Thrombosis Treatment

To find a clot and check its response to treatment, we use special ultrasound or X-ray testing.

Here are some of the common treatments for blood clots:

  • Anticoagulants – Medicines to thin the blood
  • Thrombolytic agents – Medicines to break up clots
  • Replacement factors – Doses of natural substances that control clotting

Some children with blood clotting problems need treatment only on a short-term basis. Others may need lifelong treatment. This depends on the reason for the clots.

Your child's doctor can talk with you in more detail about your child's clots and the best way to care for them. We will also work with your child's other doctors to coordinate your child's care.

Thrombosis at Seattle Children’s Hospital

At Children's Hospital, we offer a full range of services to diagnose and treat thrombosis. If your child has a problem with clots, our doctors can give a number of treatments, based on the cause.

In many cases, parents find out their child has thrombophilia when someone else in the family gets a clot and then the child is tested.

Even if the child has never had a clot, we can provide helpful services.

For example, we can help assess the child's risk for a clot, counsel the family about the effects of thrombophilia, and suggest steps to reduce the chance of clots in high-risk situations, such as surgery.

Read more about our experience and treatment of blood diseases through our Hematology Program.