Menopause App

Digital Packs Banner Digital Packs Banner

Expert Chat

What really causes blood clots? Leading vascular surgeon Professor Mark Whiteley explains


Today the MailOnline reports the story of Sarah Cobb, 27 who did of a fatal blood clot allegedly caused by the contraceptive pill.  We spoke to Professor Mark Whiteley, an expert on thrombosis and asked him for the facts behind the condition. 

Today the MailOnline reports the story of Sarah Cobb, 27 who did of a fatal blood clot allegedly caused by the contraceptive pill. This is an issue that comes up from time to time, causing confusion no doubt amongst the 3.5 million women in the UK who take it. For that reason, Healthista spoke to a leading thrombologist, Professor Mark Whiteley about what really causes blood clots.

Thrombosis is a common condition caused by blood clots forming in an artery or a vein and costs the NHS £640 million annually in the UK. Indeed, it’s been estimated that every 5 minutes someone dies of a blood clot or deep vein thrombosis. But what exactly is it and how can it be treated and avoided? Professor Mark Whiteley tells us more.

“The fact that the blood clots is a good thing. In fact we wouldn’t survive long if our blood didn’t clot.

If blood did not clot, we would be continually bleeding. Not only during the obvious times, such as when we fall over and cut ourselves, but also from lots of little bleeding points such as in our bowel, bladder, mouth etc. We are constantly springing little leaks but thanks to our clotting system, they are plugged before we even know about them.

Thrombosis is what happens when blood clots form in a vein or artery

When a blood clot forms in the heart or an artery it can cause conditions such as a heart attack, stroke, gangrene, dead leg or a dead gut, although this is not common.

The major risk factors are smoking, high blood pressure, diabetes and high cholesterol levels. On top of this are some factors we can’t influence such as old age and being male.

However, more common than this is venous thrombosis. Blood clots are much more likely to occur in the veins because veins are much bigger and have a slower flow of blood.

50 per cent of people suffering from deep vein thrombosis won’t even know that they have it

MORE:What is Lupus? Three women describe living with a disease that can attack their entire body

Signs and symptoms 

In general 50% of people suffering from deep vein thrombosis won’t even know that they have it. however if you are showing signs you may have

  • generalised pain in the leg, usually the calf, and some swelling if the blood clotting is particularly big
  • discomfort particularly on walking deeper in the calf muscle but usually do not have any swelling
  • a tender lower leg and swelling
  • if there are in the upper thigh or even pelvis, the leg is often very swollen, very blue and very           tender.

Superficial vein thrombosis (which is also called phlebitis) occur in the superficial veins and so they are closer to your skin and rarely include swelling. Symptoms may include

  • tender lumps palpable under the skin
  • they often feel like “tubes” or lumps
  • red and inflamed skin
  • inflammation around the affected area

Here are the most common causes of venous thrombosis

1. Smoking

The cure for this is simple – Stop smoking before this damage occurs.

Smoking causes “hardening of the arteries”. Although nothing is visible to the naked eye, research has shown that the vein wall in smokers is very inflamed and damaged.

Professor Whiteley says smoking whilst taking a contraceptive pill will make you especially susceptible

People who smoke have much thicker blood. Certain proteins, particularly fibrinogen, increase in smokers making the blood thicker, but as the lung tissue gets damaged, the body reacts by producing more red blood cells and this also thickens the blood.

The thickening of the blood and inflammation of the vein wall  both increase the risk of blood clots and venous thrombosis.

What does this mean for me?

In short, if you smoke – find a way to stop.


MORE: 8 proven ways to prevent cancer

2. Oral contraceptive pill

When the first oral contraceptive pills were produced in the 1960s, they had a very high dose oestrogen content, which made deep vein thrombosis and superficial vein thrombosis much more common. This was because the oestrogen changed the composition of the blood and made it much more likely to clot.

What does this mean for me?

Over the last couple of decades, combined pills with oestrogen and progesterone compounds have reduced the dose of oestrogen considerably, and the “mini pill” has no oestrogen in it at all. The result of this is that the oral contraceptive pill does slightly increase the risk of deep vein thrombosis and superficial vein thrombosis, but by itself, is only a small risk factor nowadays.

Women taking the pill may be at a higher risk

However this is completely changed if the person on the oral contraceptive pill has another risk factor which can then magnify the effect of the pill. The most important ones are smoking and being on the pill, which gives a much higher risk of thrombosis compared to the normal person. Other factors that could worsen the risk of someone on the pill include having major surgery, major illness or a family history of venous thrombosis. Women taking the oral contraceptive pill who smoke should seriously consider stopping smoking as soon as possible.

MORE: Expert: Which pill is for me?

3. Varicose Veins and “hidden varicose veins”

For decades, doctors and nurses thought that varicose veins are “only cosmetic”.

If you see bulging veins on the surface of your skin, they are varicose veins. They actually occur because the underlying veins have lost their valves and are allowing blood to fall backwards down the veins, stretching the veins below them.

Blood falling the wrong way down the veins causes inflammation around the ankles and damages the vein wall – and this can cause all sorts of problems including blood clots, bleeding, venous eczema, skin damage and even venous leg ulcers however, in many cases this will not also present bulging veins on the surface, and instead are ‘hidden varicose veins’.


Many medical professionals will tell you that if you get a clot in varicose veins, it is called “phlebitis” and does not cause any problem apart from a painful lump. Until 2012, this was widely thought to be correct. However, two major committees produced guidelines to the contrary after research had shown that clots in the superficial veins could extend into the deep veins and then fly off to the lung – a condition called pulmonary embolism or PE (an embolism of the clot in the leg veins travelling up through the veins, through the right heart and into the lungs).

What does this mean for me?

Since 2012, the guidelines state that people with “phlebitis” (or more correctly superficial venous thrombosis) should be referred to a vein clinic or a vascular service for a venous duplex ultrasound scan to check whether the clot is near the deep veins. If it is, the patient needs to have anticoagulation to reduce the risk of a pulmonary embolism. However, currently virtually nobody gets referred for such a scan and most only receive painkillers and support stockings. Incredibly, some even get given antibiotics even though there is no infection involved!

4. Operations

Having surgery is a well-recognised cause of deep vein thrombosis. This is particularly common if the surgery is of the abdomen or of a major joint such as the hip replacement.

This is due to a number of contributing factors, such as the fact that patients are usually starved for a minimum of six hours before the operation, are usually quite dehydrated and lie still for a long period of time due to general anaesthetic. The operation itself can have an impact on the major veins – either by direct pressure being placed on the major veins, such as in abdominal surgery, or twisting the veins in the legs during manipulation, as in hip surgery.


What does this mean for me?

We have now changed to performing local anaesthetic walk-in, walk-out surgery as much as possible which decreases the risk of blood clotting. As technology continues to advance, increasing numbers and types of procedures will be treated in a minimally invasive way.

5. Long distance flying

The risks of getting a deep vein thrombosis from flying hit the headlines about 15-17 years ago when a young lady flying back from Australia to the UK died after the flight. Getting deep vein thrombosis from flying was labelled “economy class syndrome” as it was instantly thought that it was the lack of movement in airline seats which caused the change in blood flow.

Research over the years has pointed out that when aircrafts fly at commercial levels, the passenger cabins are pressurised to keep the atmosphere reasonable. However, to save fuel and to keep the planes as light as possible, cabins are pressurised to somewhere around 7000 feet altitude pressure – not the full atmospheric pressure that you are used to the ground.

This means that the air is “thinner”. Scientifically, although the percentage of oxygen in the air is the same as at ground level (approximately 21%), the actual molecules are further apart due to the lower pressure and so there is less oxygen in any set volume of air. Hence each breath has got less oxygen in it than a breath at ground level. This means that the level of oxygen in the blood is decreased at altitude.


During flights under our hours this seems to have very little effect, except for people who have heart or respiratory disease. However for long flights, the low oxygen levels in the blood mean that the cells on the inside of the vein walls start undergoing a stress reaction. Rather than being perfectly flat, they start bulging a little in the middle, getting a bit lumpy and pulling apart from the cells around them.

This allows water to leak out of the blood through the vein wall and into the tissues causing swelling of the ankles.

one in 12 people can show a small deep vein thrombosis after a flight of seven hours or more

Finally, it is likely that activity is reduced and hence there is less blood pumping into your veins than there would be if you were walking around normally. This change in the blood flow can also cause clots.

A recent study has suggested that up to 1 in 12 people can show a small deep vein thrombosis after a flight of seven hours or more. Obviously most of these people do not have any obvious clinical symptoms of a deep vein thrombosis, but the fact this can be seen shows that the problem is real.

What does this mean for me?

The best ways to avoid a deep vein thrombosis in commercial flights is to consider the following: reduce the changes in your blood constituents by keeping hydrated. If you drink caffeine or alcohol (both of which dehydrate you) make sure you also drink plenty of water or juice to rehydrate.

There is precious little that can be done to reduce changes to the wall itself however, changes in the blood flow can be improved by wearing well fitted travel stockings as well as keeping your feet moving, either by wiggling them in your chair or walking up and down the aisle occasionally.

Properly fitted travel stockings reduces the amount of water that can escape from the veins into the tissue. They also narrow the veins, making the flow faster and clotting less likely. However, make sure they are properly fitted, rather than straight off the shelf or they might not have the same effect.

MORE: 5 ways to lose weight fast AND safe

6. Inactivity

Inactivity is a major cause of deep vein thrombosis. Most people will naturally move, even when asleep, due to internal body reflexes, however when we concentrate for long periods of time we can overcome these and stay stationary allowing blood to pool and therefore clot. This was the cause of death in a young man that was reported in the national press a few years ago who was playing videogames for long periods of time without moving.

Of course this is also the cause of deep vein thrombosis in people who are unable to move normally due to general immobility, plaster casts on the legs or who are unwell. In patients who are unwell, they may have additional problems such as dehydration, or the presence of cancer which also increase the risk of thrombosis.


What does this mean for me?

The best way to prevent this is to exercise. This may be as simple as just getting up and walking around every half hour or so. For those who are immobile, it is often sensible to have a thromboembolic disease compression stocking (TED stocking) or even a mechanical aid to pump the blood in the foot or lower leg, to keep blood flowing up the veins periodically. For those at very high risk, injections of heparin or tablets that reduce blood clotting (anticoagulants) can also be used to reduce risk of deep vein thrombosis.

7. Dehydration

Dehydration is a potent cause of deep vein thrombosis, as it clearly changes the constituents of the blood. Many people who are on holiday in very hot climates are particularly prone to dehydration. This can be worsened if they play sport, fall asleep on the beach and therefore don’t rehydrate, or drink a lot of alcohol/ caffeine which will also increase dehydration and the concentration of the blood.

When you are on holiday, dehydration followed by a flight home, can be a common cause of deep vein thrombosis.

When you are on holiday, dehydration followed by a flight home, can be a common cause of deep vein thrombosis.

Fever can also be a cause of dehydration. Even quite “minor” ailments such as the flu or a severe cold, that raise the temperature for a day or two, can keep a person in bed, change their mobility and reduce their access to water. If they have a temperature and are sweating a lot, they can also quickly become dehydrated.


What does this mean for me?

Any illness that causes excessive vomiting or diarrhoea will also cause severe dehydration and if you are ill and unable to rehydrate by drinking fluid, may even need to be admitted to hospital to have fluid administered by a drip.

8. Cancer

Although doctors have known that cancer can increase the risk of deep vein thrombosis and superficial venous thrombosis for many years, it has not ever been regarded as a major cause of blood clots. However, in recent years the National Institute of Health and Clinical Excellence (NICE) has highlighted that any patient who suffers from a deep vein thrombosis that occurs for no known reason, must be screened for a hidden cancer. Indeed, it is estimated that if the patient is over 40 and has an unprovoked DVT thrombosis, there is a one in 10 chance that they have a malignancy.

Anyone who has an unexpected deep vein thrombosis, particularly if they are over 40, must be examined by a doctor to look for evidence of malignancy and simple screening tests should be performed to make sure there is no hidden cancer.


What does this mean for me?

Patients with established diagnosis of cancer should be monitored for any signs of deep vein thrombosis, and if at high risk, preventative measures such as injections to reduce blood clotting or thromboembolic disease stockings should be recommended.

9. Family History

Last, but certainly not least, is a family history of blood clots in the veins. Some families have a terrible history of blood clots.

Clots in the arteries causing stokes, heart attacks, aneurysms or leg bypasses are arterial and do not count as far as this risk is concerned.

dThere are many proteins in the blood that are involved in the normal process of making or breaking down blood clots. In some families, a mutation occurs and the protein does not function normally.


What does this mean for me?

Although science is advancing all of the time, it must be pointed out that not everybody with a thrombophilia get a positive blood test. Probably over half of the families that have obvious thrombophilia will actually have a negative blood test. It is not that the blood test is wrong, it is just that we do not know all of the factors that cause thrombophilia.

Therefore anyone who has a clear family history of thrombophilia will be treated on the family history regardless of whether the blood test is positive or negative.

Treatment for blood clots 

The best way to treat these is to avoid them in the first place. That is, do not smoke, keep an eye on your blood pressure, keep any diabetes under control, exercise well, and eat well to keep your cardiovascular system healthy and your weight normal and take statins if your cholesterol is high.

However, that’s not always possible. If you do get an arterial thrombus or embolus, then treatment needs to be immediate, and depends very much on which part of the body is affected. If the clot has gone to the brain or heart then the treatment is often injecting thrombolysis to break down the clot. If the clot has gone down the leg or arm, then surgery is often required to remove the clot.




Professor Mark Whiteley is a leading vascular surgeon and founder of the Whiteley clinic. He specialises in varicose veins and performed the first endovenous “keyhole” surgery for varicose veins in the UK . Professor Whiteley has been a lecturer in surgery at Oxford University and runs regular courses for thread vein and varicose vein treatments at The Clinical Exchange.




MORE: ‘I can’t get over my affair’

MORE: 23 Pumpkin recipes to make your Halloween tastier




Like this article? Sign up to our newsletter to get more articles like this delivered straight to your inbox.

More Healthista Content