Haemorrhoids are swellings that can occur in the anus and lower rectum (back passage).

There is a network of small veins (blood vessels) within the inside lining of the anus and lower rectum. These veins sometimes become wider and engorged with more blood than usual. These engorged veins and the overlying tissue may then form into one or more small swellings called haemorrhoids.

The exact reason why these changes occur and lead to haemorrhoids forming is not clear. Some haemorrhoids seem to develop for no apparent reason. However, it is thought that the pressure in and around the anus can be a major factor in many cases. If the pressure in and around the anus is increased, then it is thought that this can lead to haemorrhoids developing.

Certain situations increase the chance of haemorrhoids developing:

Constipation- passing large stools (faeces), and straining at the toilet. These increase the pressure in and around the veins in the anus and seem to be a common reason for haemorrhoids to develop.

Pregnancy- Haemorrhoids are common during pregnancy. This is probably due to pressure effects of the baby lying above the rectum and anus, and the affect that the change in hormones during pregnancy can have on the veins.

Ageing- The tissues in the lining of the anus may become less supportive as we get older.

Hereditary factors- Some people may inherit a weakness of the wall of the veins in the anal region.