A hematoma forms when blood leaks from a damaged blood vessel and pools in a confined space inside your body, creating a pressurized pocket that can push against surrounding tissues. Unlike a simple bruise, which is flat discoloration under the skin, a hematoma produces a raised, often firm mass you can feel. The causes range from obvious trauma to medications, nutritional gaps, and underlying health conditions.
How a Hematoma Forms
Your body has a built-in repair system called hemostasis. When a blood vessel breaks, clotting factors rush to seal the opening. A hematoma develops when bleeding continues faster than the body can patch the damage, or when blood escapes into a closed space with no way to drain. That trapped blood accumulates, clots, and forms a distinct lump rather than spreading thinly through the tissue the way a bruise does.
The key distinction is structure. A bruise (ecchymosis) is flat, non-raised discoloration where a small amount of blood seeps into the skin layers. A hematoma creates a fluctuant, elevated mass under the skin. This matters because hematomas can compress nerves, restrict blood flow, or become infected, especially when they’re large or located near vital structures.
Trauma and Blunt Force Injury
Physical injury is the most common cause. Blunt force trauma from falls, car accidents, contact sports, or even bumping into furniture can rupture blood vessels beneath the skin without breaking the surface. Because the bleeding has nowhere to exit, blood collects internally. Fractures are particularly prone to causing hematomas because broken bone edges slice through nearby vessels, and the resulting bleed can be substantial.
The location of the impact matters. A hit to the shin, where there’s little tissue cushioning between skin and bone, can produce a noticeable hematoma quickly. Head injuries are especially concerning because even a moderate blow can cause blood to pool between the skull and the brain, forming subdural or epidural hematomas that create dangerous pressure.
Surgery and Medical Procedures
Hematomas are one of the most common complications after surgery. During any procedure, cutting through tissue means severing small blood vessels. If those vessels continue to bleed after the wound is closed, blood fills the empty spaces left behind. Inadequate layered closure of the surgical site, where “dead space” remains between tissue layers, gives blood a place to accumulate.
The risk is greatest in the first 48 hours after surgery. In one study of over 1,300 skin cancer surgeries, 40% of all complications were bleeding-related, with hematomas accounting for the majority. Several factors raise your odds: older age, elevated blood pressure at the time of surgery, blood-thinning medications, and high activity levels too soon after the procedure. Even routine operations like dental extractions, joint replacements, and cosmetic procedures carry hematoma risk.
Blood-Thinning Medications
Medications that reduce your blood’s ability to clot significantly increase hematoma risk. Anticoagulants like warfarin work by slowing the production of clotting factors, so even minor vessel damage can lead to prolonged bleeding beneath the skin. Antiplatelet drugs like aspirin prevent blood cells from sticking together to form clots.
The risk climbs sharply with combinations. Adding aspirin to warfarin raises hemorrhage risk by 2.5 times compared to warfarin alone. Triple therapy, combining warfarin with aspirin and a second antiplatelet drug, more than triples the risk of both non-fatal and fatal bleeding events. NSAIDs like ibuprofen and naproxen also interfere with clotting and should be used cautiously if you’re already on anticoagulants. One documented case of severe post-surgical hemorrhage was traced directly to naproxen use.
Spontaneous Hematomas Without Injury
Not every hematoma follows an obvious injury. Spontaneous hematomas can develop from internal forces or weakened blood vessels. Forceful coughing, vomiting, or straining can generate enough pressure to tear small vessels in the abdominal wall. Pregnancy, obesity, high blood pressure, and hardening of the arteries all weaken vessel walls and make this more likely.
Vascular conditions play a role too. Aneurysms, where a blood vessel wall balloons outward and thins, can rupture without warning. Malformed blood vessels that didn’t develop properly are fragile and prone to spontaneous bleeding. In these cases, you may not recall any injury at all, which can make the sudden appearance of a painful lump alarming.
Bleeding Disorders and Liver Disease
People with inherited bleeding disorders bruise and bleed more easily because their blood lacks the normal complement of clotting proteins. Hemophilia, where specific clotting factors are missing or deficient, can cause deep tissue hematomas from minimal or no apparent trauma. Von Willebrand disease, the most common inherited bleeding disorder, similarly impairs the blood’s ability to form stable clots.
Liver disease is another major contributor. Your liver produces most of the proteins involved in clotting, so when liver function declines from cirrhosis, hepatitis, or chronic alcohol use, clotting becomes unreliable. This also makes anticoagulant medications harder to control, compounding the risk. People with liver disease on blood thinners face a particularly high chance of hematoma formation from even routine daily activities.
Vitamin Deficiencies That Weaken Blood Vessels
Nutritional gaps can quietly set the stage for hematomas. Vitamin C is essential for building collagen, the structural protein that gives blood vessel walls their strength. Without enough vitamin C, collagen synthesis breaks down at the molecular level: the crosslinking that holds collagen fibers together fails, and vessel walls become fragile. The result is blood seepage that can occur in almost any organ, including deep spaces like the area behind the abdominal cavity.
Severe vitamin C deficiency (scurvy) is rare in developed countries, but moderate deficiency is not. People with restricted diets, malabsorption conditions, or heavy alcohol use are at higher risk. Vitamin K deficiency also contributes, since vitamin K is required for the liver to manufacture several key clotting factors. Without it, your blood simply takes too long to clot after a vessel is damaged. Leafy greens, fermented foods, and adequate fruit intake help maintain both of these nutrients at protective levels.
Age and Fragile Blood Vessels
Older adults develop hematomas more easily for several overlapping reasons. Skin thins with age, removing the protective cushion around blood vessels. The vessels themselves lose elasticity and become more brittle. At the same time, older adults are more likely to be taking anticoagulants or antiplatelet medications for heart conditions. Combine these factors with the balance problems and falls that become more common after 65, and the result is a population that develops hematomas from impacts that wouldn’t leave a mark on a younger person.
Post-surgical hematomas also increase with age. Slower healing, thinner tissues, and a higher likelihood of blood pressure spikes during recovery all contribute. If you’re older and notice new, unexplained lumps or deep bruising that seems disproportionate to any bump you remember, that pattern is worth mentioning to your doctor, particularly if you’re on blood-thinning medications.