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Why blood clots happen: the risk factors explained | CION Cancer Clinics
A blood clot usually forms when two or more things happen together: blood slows down, a vein wall is damaged, or the blood clots more easily than normal. Surgery, long spells without moving, cancer, pregnancy, hormone tablets and inherited clotting conditions are the common triggers. Sudden breathlessness or chest pain needs 108 now. This page explains each risk factor, which ones pass, and what it cannot tell you about your own risk. At CION Cancer Clinics, our haematologist cares for anaemia, bleeding, clotting and inherited blood disorders, with ArogyaSri, CGHS and cashless insurance accepted.
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Medically reviewed by Dr. Basudev PokhrelConsultant Haematologist · last reviewed September 2026, next review due September 2027
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On this page
- Why does a blood clot form in the first place?
- What are the common risk factors for blood clots?
- Is your risk factor temporary or lasting?
- What do families get wrong about clot risk?
- How can you lower your risk when a trigger is coming up?
- What can this page not tell you about your own risk?
- Common questions about why clots happen
The short answer
Why does a blood clot form in the first place?
A clot forms when blood that should keep moving slows down, when the wall of a vein is damaged, or when the blood itself clots more easily than it should. Most clots in the legs and lungs need two or more of these at the same time, and your risk factors are simply the things that push one of them along.
Clotting is a normal repair job
When you cut your finger, your blood clots to seal the wound. That is healthy. The trouble starts when the same repair system switches on inside a vein where there is no wound to seal. The clot then blocks the vein. Doctors call this thrombosis. A clot in a deep vein of the leg is a deep vein thrombosis, or DVT. A piece that breaks off and travels to the lungs is a pulmonary embolism.
Why one risk factor is rarely the whole story
Plenty of people take a long bus journey, or have surgery, or are pregnant, and never get a clot. What usually tips the balance is a combination. Surgery plus days in bed plus a cancer is a very different situation from surgery alone. This is why your doctor asks about your whole picture, not only the event that happened just before the clot.
This page explains risk. It cannot tell you whether you have a clot now. If you have symptoms, see the red flags below first.
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Signs that cannot wait
Sudden breathlessness, chest pain that is worse when you breathe in, coughing up blood, or fainting can mean a clot has reached the lungs. Call 108 or go to the nearest emergency department now. Do not wait to see if it settles. A painful, swollen, warm calf or thigh on one side also needs a doctor the same day. Do not rub or massage the leg, and do not start or stop any blood thinner on your own.
Not sure whether this applies to you?
Ask an oncologistThe main causes
What are the common risk factors for blood clots?
They fall into a few groups. Most people who get a clot have something from more than one group.
Surgery and hospital stays
Big operations, especially on the hip, knee, belly or pelvis, damage tissue and keep you lying still. Any stay in hospital for a serious illness raises the risk too.
Risk is higher with
- Joint replacement surgery
- Long operations under general anaesthetic
- Being confined to bed afterwards
Not moving for long
Your calf muscles squeeze blood back to the heart when you walk. When they rest for long, blood pools in the leg veins. Long journeys or a plaster cast do this.
Cancer and its treatment
Some cancers release substances that make blood clot more easily. Some chemotherapy and hormone medicines add to that, and so do central lines placed in a vein for treatment.
Hormones and pregnancy
Pregnancy and the weeks after delivery raise clot risk. So do oestrogen-containing birth control pills and some hormone replacement treatments.
Stopping a prescribed pill is a decision for your doctor, not something to do after reading this.Clotting conditions
Some people inherit a tendency to clot, such as factor V Leiden. Others develop one, such as antiphospholipid syndrome, where the immune system makes the blood sticky.
Everyday health
Older age, carrying extra weight, smoking, a previous clot, heart failure, kidney disease and inflammatory bowel disease all add some risk, usually alongside another trigger.
Side by side
Is your risk factor temporary or lasting?
Temporary triggers
Lasting risk factors
Recent surgery or an injury such as a broken leg
A cancer that is still active or under treatment
A long journey or a plaster cast
An inherited or acquired clotting condition
Pregnancy and the weeks after delivery
Long-term illness that keeps you less mobile
A hospital stay for an infection or other illness
A previous clot with no clear trigger
Usually a shorter course of blood thinner
Your team may discuss a longer course
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Commonly believed
What do families get wrong about clot risk?
"Only old people get blood clots."
Risk does rise with age, but young people get clots too. Pregnancy, the pill, surgery, a sports injury in a cast or an inherited clotting condition can all cause a clot in someone in their twenties or thirties. Leg swelling in a young person still deserves a proper check.
"A clot means the blood is too thick, so drink more water and it will clear."
Staying well hydrated is sensible, especially on long journeys. It does not dissolve a clot that has already formed. A clot needs a doctor, a scan and usually a blood thinner chosen by your treating team.
"If the clot had a reason, the family does not need to worry."
Usually that is right. A clot after major surgery does not normally mean your children carry a clotting condition. But a clot with no trigger, a clot at a young age, or several relatives with clots is worth mentioning to a haematologist.
"Once the swelling goes, the risk has gone."
The swelling settling is a good sign, but the reason the clot formed may still be there. Keep taking the blood thinner as prescribed and ask your doctor before any change.
What you can do
How can you lower your risk when a trigger is coming up?
Tell the team about your history
Before any operation, hospital stay or new hormone medicine, say if you or a close relative has had a clot. It changes how you are protected, and it is easy to forget to mention.
Ask about protection in hospital
Many hospitals assess clot risk on admission. Ask whether you need compression stockings, calf pumps or a preventive injection, and who decides when they stop.
Get moving as soon as you are allowed
Walking, even a few steps, works the calf muscles. In bed, pointing and flexing your feet helps.
Know the warning signs
Share the red flags above with the person looking after you. A clot often shows itself in the days or weeks after a trigger, once you are already home.
Being straight with you
What can this page not tell you about your own risk?
A list of risk factors is not a diagnosis. Having several of them does not mean you will get a clot, and having none does not mean you cannot. Only an examination and, where needed, an ultrasound or a scan can say whether a clot is there.
It cannot weigh your risks for you
Doctors use scoring tools that add up your age, your illness, your surgery and your history. The same risk factor counts for more in one person than in another. That calculation belongs to the team who knows your case, and it may change as your treatment changes.
It cannot tell you whether to be tested
Many families ask for a blood test for clotting conditions after a first clot. For most people it does not change the treatment, and a test done at the wrong time can mislead. Ask a haematologist whether testing makes sense in your situation.
When a haematologist helps
A clot with no clear reason, a clot that keeps coming back, or one in an unusual place such as the belly or brain should be reviewed by a haematologist. At CION, Dr. Basudev Pokhrel and the haematology team explain what it means for your treatment.
Bring every scan report, discharge summary and a list of the medicines you take, including any hormone tablets.Questions we are asked
Common questions about why clots happen
What is the most common cause of a blood clot?
There is rarely one cause. The most common picture is a recent operation or hospital stay combined with not moving much. Cancer, pregnancy, hormone tablets and a previous clot are other frequent contributors. Your doctor will usually look for more than one factor rather than stopping at the first obvious one.
Can a clot happen for no reason at all?
Yes. Doctors call it an unprovoked clot, meaning no clear trigger was found. That does not mean nothing caused it, only that the cause is not obvious. An unprovoked clot is often reviewed by a haematologist, because it can affect how long treatment continues and whether any further checks are sensible.
Does diabetes or high blood pressure cause leg clots?
They are much more closely tied to clots in the arteries, which cause heart attacks and strokes. Their link to clots in the leg veins is weaker. Keeping them well controlled still matters for your overall health, and your doctor will count them as part of your wider picture.
Can a long bus or train journey cause a clot?
Any long journey where you sit still can add a little risk, whether by bus, train, car or plane. For most healthy people the risk stays low. It matters more if you have other risk factors, such as recent surgery, pregnancy or cancer. Walking when you can and flexing your feet both help.
Is a blood clot hereditary?
Some clotting tendencies run in families, but most clots are not inherited. If a parent had a clot after surgery, your own risk is usually only slightly affected. If several relatives had clots at a young age without a trigger, tell your doctor, who can decide whether a haematologist should see you.
Why do cancer patients get clots more often?
Cancer cells can release substances that switch on clotting. Treatment adds to it: some chemotherapy and hormone medicines, central lines in a vein, surgery and time spent in bed. The risk differs a great deal by cancer type and stage, so ask your oncologist what it means for you.
Should I stop the birth control pill after a clot?
Do not change anything on your own. Tell the doctor treating the clot that you take the pill, and bring the strip with you. Oestrogen-containing pills add to clot risk, so your doctor will usually talk to you about other options. That conversation should happen with your treating team.
Does being overweight really make a difference?
It adds some risk, particularly when combined with surgery, pregnancy or long periods of sitting. It is one of the few risk factors you can change. Losing weight gradually and staying active helps, but it does not replace the protection your doctor recommends around an operation or a hospital stay.
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Sources
- NHS — Blood clots
- National Heart, Lung, and Blood Institute — Venous Thromboembolism
- American Society of Hematology — Blood Clots
- Cancer Research UK — Blood clots and cancer
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
DVT Explained: Symptoms and UrgencyHow to recognise a leg clot and how fast to actThrombophilia Testing: Who Actually Needs ItWhether a clotting blood test makes sense for youCancer-Associated ThrombosisWhy clots are more common during cancer treatmentBlood cancer risk factorsThe wider guide to risk factors for blood cancersClotting, Thrombosis and AnticoagulationEvery page in this guide in one place
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