A blood clot is a clump of blood that’s changed from a liquid to a gel-like or semisolid state. Clotting is a necessary process that can prevent you from losing too much blood when you have a cut, for example.

When a clot forms inside one of your veins, it won’t always dissolve on its own. This can be a very dangerous and even life-threatening situation.

An immobile blood clot generally won’t harm you, but there’s a chance that it could move and become dangerous. If a blood clot breaks free and travels through your veins to your heart and lungs, it can get stuck and prevent blood flow. This is a medical emergency.

Medical emergency

A blood clot may be a medical emergency. If you think you have one and experience the symptoms below, go to the nearest emergency room or emergency care clinic to be evaluated by a medical professional.

It’s possible to have a blood clot with no apparent symptoms. When symptoms do appear, some of them are the same as the symptoms of other conditions.

There’s no way to know whether you have a blood clot without medical guidance. If you know the most common symptoms and risk factors, you can give yourself the best shot at knowing when to seek an expert option.

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Illustration by Maya Chastain.

The most common place for a blood clot to occur is in your lower leg.

A blood clot in your leg or arm can have various symptoms, including:

Your symptoms will depend on the size of the clot. That’s why you might not have any symptoms or only have minor calf swelling without a lot of pain. If the clot is large, your entire leg could become swollen with extensive pain.

It’s not common to have blood clots in both legs or arms at the same time. Your chances of having a blood clot increase if your symptoms are isolated to one leg or one arm.

Blood clots can happen anywhere in the body and cause severe problems. But when they occur in the heart, the clots can cut off critical blood flow and result in a heart attack.

The heart is a less common location for a blood clot, but it can still happen. A blood clot in the heart could cause symptoms like:

When clots develop in the heart itself, it’s called coronary artery thrombosis. These clots usually occur when fatty tissues that form in the heart’s arteries break off and block blood flow to the cardiac tissues. When this blood flow is stopped, the heart tissues can’t get oxygen and other nutrients.

Blood clots that develop in the abdomen can target a variety of organs, so symptoms vary from person to person. Some people may not develop — or notice — any symptoms at all. Clots that develop in the abdomen are a form of deep vein thrombosis (DVT) and can cause symptoms like:

While these symptoms can signal a clot, they can also develop with other conditions. Before diagnosing you with an abdominal blood clot, a doctor may want to rule out other causes, like a stomach virus or food poisoning.

A blood clot in the brain is also known as a stroke.

These blood clots can develop anywhere in the body, or directly in the brain. When this happens, blood can not bring oxygen to your brain, resulting in hypoxia. Brain tissue can’t survive without a constant supply of oxygen, and hypoxia can cause severe symptoms and even death.

A blood clot in your brain will cause all the symptoms of a stroke, like:

  • paralysis
  • numbness or weakness in the arm, face, and leg, especially on one side of the body
  • trouble speaking or understanding others
  • slurred speech
  • confusion, disorientation, or lack of responsiveness
  • sudden behavioral changes, especially increased agitation
  • vision problems, like trouble seeing in one or both eyes with vision blackened or blurred, or double vision
  • trouble walking
  • loss of balance or coordination
  • dizziness
  • severe, sudden headache with an unknown cause
  • seizures
  • nausea or vomiting

If these symptoms appear and disappear suddenly, you should still seek emergency care. Stroke symptoms that come and go can be a sign of a transient ischemic attack, or ministroke. These are also usually caused by blood clots, but the clots resolve or don’t entirely block the flow of blood to your brain.

A blood clot that travels to your lungs is called a pulmonary embolism (PE). Symptoms that could be a sign of a PE are:

Your circulatory system is made up of blood vessels called veins and arteries, which transport blood throughout your body. Blood clots can form in veins or arteries.

Arterial clot

When a blood clot occurs in an artery, it’s called an arterial embolism. This type of clot causes symptoms immediately and requires emergency treatment. The symptoms of an arterial clot include:

  • a cold feeling in the affected area
  • decreased or no pulse in your arm or leg
  • paralysis or lack of movement in the affected area
  • pain
  • a pale color in the arm or leg
  • weakness
  • blisters on the skin around the affected artery
  • shedding of the skin
  • skin erosions or ulcers
  • discoloration or damage (necrosis) of the skin around the affected artery

Venous clot

A blood clot that occurs in a vein is called a venous embolism. These types of clots may build up more slowly over time, but they can still be life-threatening. Symptoms of a venous blood clot include:

  • swelling
  • pain or tenderness
  • increased warmth
  • cramps or aching
  • red or discolored skin

The most serious type of venous clot is DVT. With DVT, a clot forms in one of the major veins deep inside your body. It’s most common for this to happen in one of your legs, but it can also happen in your:

  • arms
  • pelvis
  • lungs
  • brain

The Centers for Disease Control and Prevention (CDC) estimates that DVT, together with pulmonary embolism (a type of venous clot affecting the lungs), affects up to 900,000 Americans each year. These types of blood clots kill approximately 100,000 Americans annually.

Several things can cause blood clots, and the cause usually depends on what type of clot it is.

When blood clots develop in your arteries, they’re usually caused by bits of plaques — made up of fat or mineral deposits — that break off and block blood flow.

Clots that form in the veins have a wider variety of causes, like:

  • a disease or injury to the area where the clot forms
  • immobility or lack of movement
  • a broken bone
  • inherited or genetic disorders that affect how your blood clots
  • obesity
  • autoimmune disorders
  • certain medications, like birth control or hormone therapy

Certain risk factors increase your chances of having a blood clot. A recent hospital stay, especially one that’s lengthy or related to major surgery, increases your risk of a blood clot.

Common factors that can put you at a moderate risk for a blood clot include:

  • age, especially if you’re over 65 years old
  • lengthy travel, like any trips that caused you to sit for more than 4 hours at a time
  • bed rest or being sedentary for long periods of time
  • obesity
  • pregnancy
  • a family history of blood clots
  • smoking
  • cancer
  • certain birth control pills

Children can get blood clots, but they’re more common in children who are hospitalized.

About 1 in 10,000 children overall are diagnosed with blood clots, compared to 1 out of every 200 hospitalized children. Reduced movement due to illness or injury is a big source of blood clots in hospitalized children, but many also face congenital or genetic disorders present at birth.

Some of the common causes of blood clots in children are:

  • reduced blood flow
  • damage to veins from intravenous catheters
  • inherited conditions like genetic thrombophilia
  • abnormal blood vessel structure or formation, like May-Thurner Syndrome and Paget-Schroetter Syndrome
  • certain medications

Diagnosing a blood clot by symptoms alone is very difficult. According to the CDC, almost 50 percent of people with DVT have no symptoms.

You should call your doctor immediately if you think you might have a blood clot. A healthcare professional will look at your symptoms and medical history and let you know what steps to take from there.

Your doctor or other healthcare professional will be able to tell whether there’s a reason for concern and can send you for more tests to determine the exact cause.

In many cases, the first step will be a noninvasive ultrasound. This test will show an image of your veins or arteries, which can help your doctor make a diagnosis.

Medical emergency

Symptoms that come out of nowhere are especially concerning. Call 911 or go to the nearest emergency room immediately if you experience any of the following:

  • sudden shortness of breath
  • chest pressure
  • difficulty breathing, seeing, or speaking