Blood clots form when blood cells and factors in the blood thicken, forming a semi-solid mass. 

Sometimes, if the blood clot is caused by an injury, then it is considered protective, since it will plug the injured vessel to stop bleeding. But when blood clots form in deep veins of the leg or arm, in the absence of injury, they can be dangerous. 

The two most common types of blood clots are deep vein thrombosis (DVT) and pulmonary embolism (PE), which are often grouped together and referred to as venous thromboembolism. Here's what you need to know about the causes and risk factors for DVT and PE. 

What causes DVT and PE

DVT is a clot that can form in a deep vein in the lower leg or thigh, but also your pelvis or arms. PE happens when a clot detaches from a deep vein and travels through the heart to the lungs, which causes a blockage and prevents adequate blood flow to the lungs. This happens in about 40% to 50% of patients with DVT. 

"The most common place for clots to travel is the lungs, which can cause breathing troubles and affect the heart," says Danielle Bajakian, MD, a vascular surgeon at Columbia University Irving Medical Center. 

In fact, the Centers for Disease Control and Prevention (CDC) estimate that close to 900,000 Americans are affected by DVT and PE each year — and between 60,000 and 100,000 will die from these conditions.  

Major risk factors for DVT and PE 

According to Bajakian, some of the most common risk factors of DVT and PE include: 

• Surgery. According to the CDC, about half of all blood clots happen within three months of surgery. A 2011 review published in the journal ISRN Hematology found that major general surgery, major trauma, spinal cord injury, hip or knee replacement, and a fracture to the hip or leg are often associated with DVT and PE. 
• Immobility. Extended bed rest due to illness or recent surgery, long plane or car rides, and a sedentary lifestyle are all known to cause slow blood flow, which can lead to clotting. 
• Pregnancy. Compared to non-pregnant women, pregnant women are five times more likely to have blood clots, according to the CDC. During the postpartum period, blood clots are 20 times more likely when compared to non-pregnant women, according to a 2009 review published in the journal Arteriosclerosis, Thrombosis, and Vascular Biology. 
• Medications. Hormone replacement therapy and oral contraceptives (birth control pills) can increase the risk of DVT for some people. This happens because of the changes in the hemostatic balance, or the components in your blood, after taking hormones. 
• Family history. You may have more than double the risk of developing a blood clot if you are a child or sibling of someone who had a blood clot, according to a 2009 study published in JAMA Internal Medicine.   
• Prior blood clots. A 2007 study published in the journal Haematologica found that about 25% of patients with DVT or PE will experience recurrent venous thromboembolism (VTE) within five years.
• Smoking. A 2013 meta-analysis published in PloS Medicine, found a slightly increased risk of VTE (includes both DVT and PE) for smokers compared with non-smokers.

How to know if you have a blood clot

The symptoms of blood clots can vary in severity. According to the CDC, sudden death is the only symptom in about 25% of PE cases. 

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But in general, Bajakian says those with blood clots will feel other symptoms beforehand. These often include swelling, pain, or redness in the arm or leg, and may be accompanied by unexplained shortness of breath, chest pain, or rapid heart rate. 

If you are experiencing any of these symptoms in conjunction with the risk factors, Bajakian says you should seek immediate medical attention. 

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