The aorta is the largest blood vessel in the body. It transports oxygen-rich blood away from the heart to the rest of the body.

An abdominal aortic aneurysm (AAA) occurs when part of the aorta wall becomes weakened and the large amount of blood that passes through it puts pressure on the weak spot, causing it to bulge outwards to form an aneurysm.

The abdominal aorta is usually around 2cm wide – about the width of a garden hosepipe  – but can swell to over 5.5cm, which is what doctors classify as a large aneurysm.

Risk factors for an AAA

It's not known exactly what causes the aortic wall to weaken, although increasing age and being male are known to be the biggest risk factors.

One study found that people aged over 75 are seven times more likely to be diagnosed with an AAA than people under 55 years old.

Men are around six times more likely to be diagnosed with an AAA than women.

However, there are other risk factors that you can do something about – described below – the most important of which is smoking.

Smoking

Research has found that smokers are seven times more likely to develop an AAA than people who have never smoked.

The more you smoke, the greater your risk of developing an AAA. People who regularly smoke more than 20 cigarettes a day may have more than 10 times the risk of non-smokers.

The risk may increase because tobacco smoke contains harmful substances that can damage and weaken the wall of the aorta.

Atherosclerosis

Atherosclerosis is a potentially serious condition where arteries become clogged up by fatty deposits, such as cholesterol.

An AAA is thought to develop because these deposits (called plaques) cause the aorta to widen in an attempt to keep blood flowing through it. As it widens, it also gets weaker.

Smoking, eating a high-fat diet and high blood pressure all increase your risk of developing atherosclerosis.

High blood pressure

As well as contributing to atherosclerosis, high blood pressure (hypertension) can place increased pressure on the aorta's wall.

Family history

Having a family history of AAAs means that you have an increased risk of developing one.

One study found that people who had a brother or sister with an AAA were eight times more likely to develop one than people whose siblings were unaffected.

This suggests that certain genes you inherit from your parents may make you more vulnerable to developing an AAA.

However, no specific genes have yet been identified.

Read about how to reduce your risk of developing an AAA.