Pulmonary embolism (PE) happens when a blood clot, usually formed in the deep veins of your legs, breaks free and travels to your lungs. The good news: most PEs are preventable. Prevention comes down to keeping blood moving, reducing clot formation in the first place, and recognizing early warning signs before a leg clot becomes a lung emergency.
How Pulmonary Embolism Develops
Most clots that cause PE start small, forming in the calf veins, often in the pockets around vein valves where blood pools. About 25% of untreated calf clots extend upward into the larger thigh veins within a week. Once a clot reaches these proximal veins, pieces can break off and travel through the bloodstream to the lungs.
Without treatment, roughly half of people with a clot in the thigh veins or lungs will have another clotting event within three months. That progression makes early prevention critical, especially during high-risk periods like the days and weeks following surgery, when the highest risk of fatal PE falls between days three and seven. The overall risk stays elevated for two to three months after a major procedure.
Who Is at Higher Risk
Hospitals use scoring systems to estimate your clot risk before surgery or during a hospital stay. The factors that raise your risk fall into a few categories:
- Age: Risk climbs with each decade, with a notable jump after 60 and again after 75.
- Immobility: Being confined to bed for more than 72 hours, wearing a cast, or having a spinal cord injury all significantly increase risk.
- Surgery: Any procedure lasting longer than 45 minutes raises risk. Hip, knee, and pelvis surgeries carry the highest levels.
- Body weight: A BMI over 25 is considered a risk factor.
- Medical conditions: Active cancer, heart failure, sepsis, pneumonia, and inflammatory bowel disease all contribute.
- Personal or family history: A previous clot or PE, or a family history of blood clots, puts you in a higher risk category.
Genetic Clotting Disorders
Some people inherit gene variants that make their blood clot more easily. The most common is Factor V Leiden. In the general population, about 1 in 1,000 people develop an abnormal clot each year. Carrying one copy of this mutation raises that to 3 to 8 in 1,000. Carrying two copies pushes the risk as high as 80 in 1,000. The risk climbs further if you have more than one clotting mutation. If you have a strong family history of blood clots, especially at young ages, genetic testing can help you and your doctor plan appropriate prevention.
Staying Active and Mobile
Blood clots form when blood sits still. That’s why the simplest and most effective prevention strategy is movement. If you’re recovering from surgery, getting up and walking as soon as your medical team allows is one of the most important things you can do. Even small movements matter when you can’t walk yet: flexing your feet, raising and lowering your toes, and tightening and releasing your leg muscles all help push blood through your veins.
For everyday life, long periods of sitting at a desk or on the couch carry a lower but real risk. Standing up and walking around periodically keeps blood circulating through the deep veins in your legs.
Prevention During Long Flights and Car Trips
Travel lasting more than four hours is a recognized risk factor for blood clots. The CDC recommends several practical steps:
- Choose an aisle seat so you can get up and walk every two to three hours.
- Exercise your calves while seated: raise and lower your toes while keeping your heels on the floor, or tighten and release your leg muscles.
- Stay hydrated and limit alcohol, which can contribute to dehydration.
- Wear compression stockings if your doctor has recommended them, especially if you have other risk factors.
These steps matter most for people who already have one or more risk factors. A healthy person on a five-hour flight has a very low absolute risk, but someone with a recent surgery, active cancer, or a clotting disorder should take these precautions seriously.
Compression Devices and Stockings
Hospitals commonly use two types of mechanical prevention. Intermittent pneumatic compression (IPC) devices are inflatable sleeves that wrap around your legs and rhythmically squeeze them, mimicking the pumping action of walking. A large meta-analysis found that IPC devices work about as well as blood-thinning medications at preventing clots, with a significantly lower risk of bleeding, about 59% lower. Combining IPC with medication reduced clot risk even further, cutting it roughly in half compared to IPC alone.
Graduated compression stockings, the kind you can buy over the counter, have a more mixed evidence base. A major trial involving over 2,500 stroke patients found that thigh-length stockings provided only a small, statistically insignificant reduction in clot risk. Thigh-length stockings did outperform below-knee stockings, reducing proximal clots by about 25 per 1,000 patients. But at six months, researchers found no significant effect on clot rates, survival, or quality of life. Compression stockings may still help certain patients, but they’re not a substitute for movement or medication in high-risk situations.
Blood-Thinning Medications After Surgery
If you’re having a major surgery, particularly hip or knee replacement, your surgical team will likely start you on a blood-thinning medication to prevent clots. The standard approach uses injectable blood thinners, sometimes followed by a low-dose oral option. For hip and knee replacements, current guidelines recommend continuing some form of clot prevention for 14 to 35 days after surgery. One common protocol uses injectable medication for the first 10 days, then switches to a daily pill for another 28 days.
The type and duration of medication depend on your surgery, your weight, your kidney function, and your overall risk profile. If you’re having surgery and haven’t been told about clot prevention, ask your surgical team what their plan is. This is a standard part of surgical care, and it should be discussed before your procedure.
Recognizing a Leg Clot Early
Catching a clot in your leg before it travels to your lungs can be lifesaving. The classic signs of a deep vein clot include:
- Tenderness along the inner leg, following the path of the deep veins
- Swelling in one leg, particularly if the calf is more than 3 centimeters larger than the other side
- Pitting edema, where pressing on the swollen area leaves a temporary dent
- Visible surface veins that weren’t there before (not varicose veins)
- Warmth or redness over the affected area
The key detail is asymmetry. Swelling in both legs is common and usually unrelated to clots. Swelling, pain, or warmth in one leg, especially after surgery, a long trip, or a period of immobility, warrants prompt evaluation. Doctors use a combination of clinical scoring, blood tests, and ultrasound to confirm or rule out a clot quickly.
Everyday Habits That Lower Your Risk
Beyond specific high-risk situations, a few baseline habits help keep your clot risk low. Regular physical activity is the most important. Exercise strengthens the calf muscles that act as pumps to move blood back up from your legs. Maintaining a healthy weight matters because a BMI over 25 is an independent risk factor. Staying well hydrated helps keep your blood flowing smoothly, since dehydration thickens blood and promotes stasis in the veins.
Hormonal medications, including birth control pills and hormone replacement therapy, increase clot risk. If you have other risk factors, especially a known clotting disorder or family history, discuss alternatives with your prescriber. Smoking also raises clot risk and compounds the effects of hormonal medications.