Blood Clots After Hip or Knee Replacement: Risk Window, Prevention and Symptoms

Deep vein thrombosis is among the most common complications after a hip or knee replacement. The prevention plan is a mix of medicine, movement and compression, and it runs well past the day you leave hospital.

Ask an orthopedic surgeon what they watch most closely after a hip or knee replacement, and blood clots will be near the top of the list. The AAOS guides to total hip replacement (OrthoInfo, 2026) and total knee replacement (OrthoInfo, 2026) both describe clots as among the most common complications, and both note that a clot that travels to the lungs can be life-threatening.

The good news is that the risk is well understood and surgeons have a standard toolkit to lower it. This guide explains why surgery raises clot risk, when that risk is highest, what prevention usually looks like, and what symptoms mean you should get help immediately. Your surgeon will set your own plan; this is background to help you follow it.

DVT and PE in plain terms

A deep vein thrombosis (DVT) is a clot in one of the deep veins that run through the muscles, most often in the calf, thigh or pelvis. A pulmonary embolism (PE) happens when part of that clot breaks away and lodges in the lungs. The AAOS page on deep vein thrombosis (OrthoInfo, 2026) calls a PE a serious medical emergency that can be fatal. It also notes a longer-term problem: a clot can damage the valves inside the vein, leading to post-thrombotic syndrome, with lasting leg pain, swelling, skin changes and sores.

Why joint replacement raises the risk

The DVT page names major hip and knee surgery among the most common orthopedic risk factors and describes three reasons, which doctors have long grouped together:

On top of the surgery itself, the same page lists general risk factors: older age, a previous clot or a family history of one, cancer, smoking, hormone therapy or birth control pills, pregnancy, obesity and inherited clotting disorders. If any apply to you, make sure your surgeon knows, because they can change how aggressive the prevention plan needs to be.

When the risk is highest

Two timing points from the OrthoInfo DVT page are worth writing on your fridge:

• Risk peaks 2 to 10 days after orthopedic surgery, a window that includes the days after you have been discharged.
• Patients remain at risk for about three months after surgery.

That first window matters for planning. Many patients assume the danger is over once they are home, when in fact the highest-risk days often fall during the first week at home or in a hotel. It is also exactly when people are tempted to travel.

What prevention usually looks like

Surgeons typically combine several measures rather than rely on one. OrthoInfo describes the following.

1. Moving early

Walking or leg exercises start as soon as possible after surgery, often the same day. Simple movements such as ankle pumps and leg lifts in bed keep blood moving through the calf. Physical therapists teach exercises for strength and range of motion that also help circulation in the deep veins, and pain relief is partly about making that movement possible.

2. Compression

Two kinds are common. Graded elastic stockings are tightest at the ankle and looser further up the leg. Pneumatic compression devices are inflatable sleeves or boots that squeeze the calf or foot in pulses. You may use the second kind in hospital and the first at home.

3. Blood thinners

Anticoagulant medication is often started the day after joint replacement and continued at home. How long you take it depends on the operation and your personal risk factors, so the duration is your surgeon's decision. Ask for it in writing, along with what to do if you miss a dose.

4. A filter, rarely

For the small number of people who cannot take blood thinners, a filter can be placed in the main vein leading to the heart to catch clots. OrthoInfo describes this as rarely used.

Symptoms that need attention

The DVT page notes that many people with a clot have no symptoms at all, which is why prevention matters so much. When symptoms do appear, they fall into two groups.

Possible clot in the leg: contact your surgical team promptly

• Swelling in the calf, ankle or foot that does not settle when you raise the leg
• Calf pain or tenderness that is separate from the incision
• Red or discolored skin
• Veins that look enlarged, or a firm, cord-like vein

Possible clot in the lungs: emergency care now

• Sudden shortness of breath
• Sudden chest pain, or chest pain that worsens when you cough
• Coughing up blood

OrthoInfo points out that a PE can be the first sign that there was a DVT at all. Do not wait to see whether chest symptoms pass.

Swelling that is normal, and swelling that is not

Some swelling after a knee replacement is expected. The AAOS guide to activities after knee replacement (OrthoInfo, 2026) describes moderate to severe swelling in the first days or weeks, easing to mild or moderate for roughly three to six months. The difference to watch for is swelling that is new, worsening, one-sided in the calf, or that does not respond to elevation, especially when it comes with calf pain. If you are unsure which you have, ask; an ultrasound answers the question quickly.

How a clot is diagnosed and treated

The usual test is a duplex ultrasound, which shows both the vein and the blood flow through it. It is painless, uses no radiation and can be repeated. MRI and contrast venography are used less often, and blood tests can help in some cases.

Treatment is usually medication rather than surgery. OrthoInfo lists several types of anticoagulant, including oral Xa inhibitors that need no routine blood monitoring, heparin given intravenously, low molecular weight heparin given by injection, and warfarin, which needs regular blood tests and is used less often now. The aim is to stop the clot growing, keep it from reaching the lungs and reduce the chance of new clots.

Clots and travel after surgery

This is where clot risk meets the practical reality of having surgery away from home. The CDC Yellow Book (2026) notes that both surgery and air travel raise clot risk independently, and that travelling soon after surgery adds risk because you sit for long periods while your blood is more prone to clot. The knee activities guide adds that pressure changes and immobility on flights can increase leg swelling, and advises asking your doctor before flying.

None of these sources gives a single safe day to fly after joint replacement, and you should be wary of anyone who quotes one without examining you. What you can do is plan around the decision:

• Keep the return date flexible until your surgeon has cleared you to fly.
• Ask which blood thinner you will be on during the flight and whether the dose timing needs to change across time zones.
• Ask whether compression stockings should be worn for the journey.
• Book an aisle seat so you can stand and walk; MedlinePlus (reviewed 2024) advice for hip patients on long car trips is to stop and walk about every two hours, and the principle of breaking up long sitting applies in the air too.
• Carry a short medical summary listing your surgery date, implant and medications in case you need care on arrival.

Questions to ask before surgery

1. Which blood thinner will I take, starting when, and for how long?
2. Do any of my medical conditions or medicines change my clot risk or the drug you would choose?
3. Will I use compression in hospital and at home?
4. What leg exercises should I do, and how often?
5. Who do I call about leg swelling at night or at the weekend?
6. When do you expect I could fly, and what would make you delay it?

The bottom line

Blood clots are a known and manageable risk after hip and knee replacement, but they demand respect for weeks, not days. The highest-risk window often falls after discharge, prevention depends on you as much as on the hospital, and chest symptoms are always an emergency. Build your recovery and any travel around your surgeon's clot plan, not the other way round.

Common questions

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