Thrombus (Blood Clot)

Nov 7
8 min read

Updated: Nov 9

Blood clots, clinically known as a thrombus, are gel-like clusters of platelets, fibrin, and red blood cells that form within arteries or veins. They serve a vital role in preventing excessive bleeding after injury or surgery, but when they form abnormally, they can obstruct blood flow and trigger severe complications such as deep vein thrombosis (DVT), pulmonary embolism (PE), stroke, or heart attack.

Overview

What is a blood clot?

Every year, blood clots kill more people than breast cancer, AIDS, and car accidents combined—yet most people have no idea how or why they form. A blood clot is a semi-solid aggregation of platelets, red and white blood cells, and fibrin protein that develops when the body senses vessel injury or disruption in normal blood flow. While this mechanism is vital for stopping bleeding after a wound, clots can also appear without trauma, triggered by conditions such as thrombophilia, cancer, infection, hormonal therapy, or prolonged immobility. When this happens, the clot may partially or completely block blood flow, cutting off oxygen to tissues and causing organ damage or death if not recognized and treated promptly.

Function

What purpose do blood clots serve in the body?

The human body forms hundreds of tiny clots every day, and in most cases, they dissolve quietly within minutes. This intricate system of coagulation and repair is what keeps a single paper cut from becoming fatal. Blood clots act as the body’s emergency response system when vessels are injured. When a vessel wall is damaged from a cut, scrape, or surgical incision, platelets rush to the site and clump together, forming a temporary plug. This activates a cascade of clotting factors that generate fibrin strands, which weave through the platelets to stabilize the clot and seal the wound. In healthy individuals, this prevents blood loss and allows the vessel to heal.

But the same system that saves lives can also turn against the body. Extended immobility, cancer, pregnancy, obesity, or inherited clotting disorders can cause blood to clot when it shouldn’t. Hormonal contraceptives and hormone therapy can heighten this risk further by altering clotting factor levels. When blood thickens or pools without injury, it can form clots that block circulation, starving organs of oxygen and triggering life-threatening complications.

Anatomy

What are blood clots made of?

Under a microscope, a blood clot looks deceptively simple—yet in reality, it is one of the body’s most complex biological structures. Within seconds of an injury, millions of microscopic platelets and strands of fibrin interlock to form a living barrier strong enough to withstand blood pressure surges of more than 100,000 heartbeats a day. Blood clots consist primarily of platelets and fibrin. Platelets are small, disc-shaped cell fragments produced in the bone marrow that circulate until they detect vessel injury. Once activated, they become sticky and adhere to the damaged surface. Fibrin, a protein formed from fibrinogen during the clotting cascade, acts as a structural mesh that binds platelets and red blood cells together, reinforcing the clot’s stability.

Microscopically, a blood clot appears as a dense, reddish mass interlaced with fibrin fibers. The red hue comes from red blood cells trapped within the fibrin network. This structure forms a durable barrier that stops bleeding until the vessel wall repairs itself, after which enzymes such as plasmin dissolve the clot through fibrinolysis.

Location

Where do blood clots most commonly develop?

Although blood clots can occur anywhere in the body, nearly 900,000 Americans each year develop one in a deep vein—most often in the legs or pelvis—and up to one-third of those cases prove fatal when the clot travels to the lungs. Blood clots can form anywhere in the circulatory system. Venous clots most often develop in the deep veins of the legs or pelvis, a condition known as deep vein thrombosis (DVT). If part of a venous clot breaks free and travels to the lungs, it becomes a pulmonary embolism (PE), a medical emergency that can block oxygen exchange and cause sudden shortness of breath or chest pain. Arterial clots, by contrast, obstruct blood flow to critical organs such as the heart or brain, resulting in a heart attack or ischemic stroke.

Symptoms

What symptoms can indicate a blood clot?

Blood clots are often called “silent killers” because up to half of all deep vein thrombosis (DVT) cases cause little or no warning before becoming life-threatening. Symptoms depend on where the clot forms and how severely it obstructs blood flow. DVT may cause swelling, warmth, redness, and a heavy, aching pain in one leg. A pulmonary embolism (PE) often presents with chest pain, rapid heartbeat, and shortness of breath. Clots in the coronary arteries can trigger crushing chest pain or radiating discomfort in the arm, jaw, or back. In the brain, an arterial clot may cause sudden weakness, slurred speech, or loss of coordination, all of which are signs of an evolving stroke that demand immediate medical attention.

Conditions and Disorders

What medical conditions affect blood clotting?

An estimated one in ten people carries a genetic or acquired condition that alters how their blood clots—yet many remain undiagnosed until a serious event occurs. Blood clots can arise as part of the body’s normal healing response, but a range of inherited or acquired disorders can disrupt this balance, leading to either excessive bleeding or an increased tendency to form dangerous clots. These conditions fall into two main categories: bleeding disorders, in which the blood cannot clot properly, and clotting disorders, in which the blood clots too easily. Both types can cause severe or life-threatening complications if not identified and managed appropriately.

Bleeding Disorders

What are the most common bleeding disorders?

Up to one in ten women experience symptoms of a bleeding disorder in their lifetime, yet most go undiagnosed—often dismissed as “heavy periods” or postpartum complications. Bleeding disorders occur when one or more components of the clotting system—platelets, clotting factors, or vessel integrity—fail to function normally. This prevents blood from clotting efficiently and can lead to excessive bleeding from menstruation, childbirth, surgery, or even minor injuries.

Von Willebrand Disease (VWD)

The most common inherited bleeding disorder worldwide, Von Willebrand Disease (VWD) affects women twice as often as men due to menstrual and obstetric bleeding, revealing its presence sooner. It results from a deficiency or dysfunction of von Willebrand factor, a protein that helps platelets adhere to damaged vessels. While usually genetic, it can also develop later in life from autoimmune disease, cancer, or cardiovascular conditions.

Thrombocytopenia

Women are more likely than men to experience thrombocytopenia during pregnancy or as a side effect of autoimmune disease or certain medications. This condition is defined by a platelet count below 150,000 per microliter of blood, limiting the body’s ability to stop bleeding effectively. It can lead to easy bruising, prolonged bleeding, or dangerous hemorrhage during childbirth.

Hemophilia

Although hemophilia is classically associated with men, many women carry the gene and can experience reduced clotting factor levels themselves. These women may face heavy menstrual bleeding, postpartum hemorrhage, or joint pain from internal bleeding—often without formal diagnosis or treatment.

Blood-Clotting Disorders (Hypercoagulable States)

What are the most common blood-clotting disorders?

Women face a unique set of risks when it comes to hypercoagulable states, conditions that make the blood clot too easily. These disorders may be inherited or acquired later in life, and many are first uncovered during pregnancy, after a miscarriage, or while taking hormonal birth control. Up to 60% of women who experience an unexplained blood clot have an underlying clotting disorder that was never previously identified.

Factor V Leiden

The most common inherited thrombophilia, this genetic mutation affects roughly 5% of women of European ancestry. It prevents Factor V from being properly inactivated by Protein C, which allows the clotting process to continue unchecked. Women with Factor V Leiden face a significantly higher risk of developing deep vein thrombosis (DVT) or pulmonary embolism (PE), particularly during pregnancy or while using hormonal contraception.

Prothrombin G20210A Mutation

This inherited mutation increases prothrombin (Factor II) production, a key protein that promotes clot formation. Carriers, especially women taking estrogen-containing medications, have a two to threefold higher risk of developing a venous blood clot. This mutation is often detected only after a first clotting event or recurrent pregnancy loss.

Antiphospholipid Syndrome (APS)

Affecting women up to nine times more often than men, antiphospholipid syndrome (APS) is an autoimmune condition in which antibodies attack phospholipids and related proteins in cell membranes, triggering clot formation throughout the body. It is a major but often overlooked cause of recurrent miscarriages, stroke, and pulmonary embolism (PE) in otherwise healthy women.

Risk Factors

What other factors increase the risk of blood clots?

Beyond inherited and autoimmune causes, several lifestyle and medical factors increase clotting risk by slowing circulation or altering blood chemistry. These include:

Age: The risk rises substantially after age 65 as circulation slows and vessel walls stiffen.
Pregnancy and Postpartum Period: Hormonal changes and increased pressure on pelvic veins elevate the risk of clotting.
Obesity: Excess weight contributes to venous stasis and inflammation that promote clot formation.
Cancer: Tumors and certain chemotherapy drugs can activate clotting pathways.
Hormonal Birth Control and Hormone Therapy: Estrogen-containing medications increase clotting factor levels and reduce natural anticoagulants.
Smoking: Damages vessel linings and enhances platelet aggregation.
Immobility: Prolonged sitting, bed rest, or long-haul travel can cause venous pooling and DVT.
Severe COVID-19 Infection: The inflammatory response and endothelial damage caused by SARS-CoV-2 significantly heighten coagulation activity, leading to widespread microthrombi in severe cases.

Prevention

Can blood clots be prevented?

Blood clots remain one of the leading causes of preventable death worldwide, yet there is still no standardized or routine screening protocol to identify people at risk before a catastrophic event occurs. Despite decades of evidence, most healthcare systems do not perform comprehensive clotting-risk evaluations prior to prescribing hormonal therapies, planning surgery, or managing pregnancy—all of which can dramatically alter coagulation. This includes all forms of hormonal therapy, not only estrogen-based treatments; progestins, testosterone, and combination regimens have each been linked to increased clotting activity in susceptible individuals.

While inherited disorders themselves cannot be prevented, most deadly clots develop from acquired or compounded factors that are entirely manageable with proper vigilance. Prevention requires a shift from reactive to proactive medicine: consistent risk assessments, informed prescribing, adequate prophylaxis during hospitalization and recovery, and patient education about symptoms and triggers. Until systematic testing and accountability are standard practice, preventable blood clots will continue to take lives unnecessarily.

Reducing Risk of Blood Clots

What can I do to lower my risk of developing a blood clot?

You can significantly lower your risk of developing clots by maintaining overall vascular health and addressing modifiable risk factors:

Regular medical care: Schedule annual checkups and discuss your family history, medications, and any symptoms of swelling, pain, or unexplained shortness of breath with your healthcare provider. Those with known clotting disorders or prior clot events may require ongoing monitoring or anticoagulant therapy.
Cancer and chronic disease screenings: Certain cancers and long-term illnesses increase the risk of clotting by promoting inflammation or damaging blood vessel linings. Early detection through regular screenings allows for proactive management.
Maintain a healthy weight: Excess weight contributes to venous stasis and increases pressure on leg veins, especially during periods of inactivity. A balanced diet and regular exercise help improve circulation and reduce inflammation.
Avoid smoking: Nicotine and other chemicals in cigarettes damage blood vessels, increase platelet stickiness, and accelerate clot formation. Quitting smoking immediately lowers vascular risk.
Stay hydrated: Dehydration thickens the blood and slows its flow, particularly during travel or illness. Aim for consistent fluid intake throughout the day.
Review hormonal options: Discuss non-estrogen alternatives for contraception or hormone replacement therapy with your clinician if you have a personal or family history of clots. Progestin-only or non-hormonal methods may be safer options.
Stay active: Regular movement keeps blood circulating efficiently. If you must sit for long periods—such as during travel or office work—stand, stretch, and walk every hour. Compression stockings can also help prevent venous pooling in the legs.

The International Women’s Blood Clot Advocates (IWBCA) provides the information and materials on this site for general educational purposes only. The content is not intended to replace professional medical evaluation, diagnosis, or treatment. Always seek the advice of your physician or other qualified healthcare provider regarding any medical condition or treatment decisions. Never disregard professional medical advice or delay seeking care because of information found on this site. If you are experiencing a medical emergency, call 911 immediately.