Varicose veins are usually caused by weak vein walls and valves.
Inside your veins are tiny one-way valves that open to let the blood through, and then close to prevent it flowing backwards.
Sometimes the walls of the veins become stretched and lose their elasticity, causing the valves to weaken.
If the valves don't function properly, this can cause the blood to leak and flow backwards. If this happens, the blood collects in your veins, which become swollen and enlarged.
The reasons why the walls of the veins stretch and the valves in your veins weaken aren't fully understood. Some people develop the condition for no obvious or apparent reason.
A number of things can increase your likelihood of developing varicose veins, including:
Women are more likely to be affected by varicose veins than men. Research suggests this may be because female hormones tend to relax the walls of veins, making the valves more prone to leaking.
Your risk of developing varicose veins is increased if a close family member has the condition.
This suggests varicose veins may be partly caused by your genes (the units of genetic material you inherit from your parents).
Visit our page on genetics for more information about how you inherit your physical and behavioural characteristics.
As you get older, your veins start to lose their elasticity and the valves inside them stop working as well.
Being overweight puts extra pressure on your veins, which means they have to work harder to send the blood back to your heart.
This can put increased pressure on the valves, making them more prone to leaking.
The impact of body weight on the development of varicose veins appears to be more significant in women.
Use the healthy weight calculator to check whether you're overweight.
Some research suggests jobs that require long periods of standing may increase your risk of getting varicose veins.
This is because your blood doesn't flow as easily when you're standing for long periods of time.
During pregnancy, the amount of blood increases to help support the developing baby. This puts extra strain on your veins.
Increased hormone levels during pregnancy also cause the muscular walls of the blood vessels to relax, which also increases your risk.
Varicose veins may also develop as the womb (uterus) begins to grow. As the womb expands it puts pressure on veins in your pelvic area, which can sometimes cause them to become varicose.
Although being pregnant can increase your risk of developing varicose veins, most women find their veins significantly improve after the baby is born.
Read more about varicose veins during pregnancy.
In rare cases, varicose veins are caused by other conditions.