Haemophilia is caused by an inherited change to a gene. It mainly affects males.

The gene change is on the X chromosome. It can be carried by either the mother or father, or both.

The chances of a child inheriting the haemophilia changed gene depends on which of their parents has the changed gene.

If only the mother has the changed gene

If a woman with the changed gene and an unaffected man have a baby, there's a:

• 1 in 4 chance of having an unaffected baby boy
• 1 in 4 chance of having a baby boy with haemophilia
• 1 in 4 chance of having an unaffected baby girl
• 1 in 4 chance of having a baby girl with an affected X chromosome

In the last situation, the girl becomes a carrier of the changed gene. This means she can pass it on to her children but will not usually have any severe symptoms of haemophilia herself.

However, some female carriers sometimes have bleeding problems, such as heavy periods.

If only the father has the changed gene

If a man with haemophilia has a son with an unaffected woman, there's no chance the boy will get haemophilia.

This is because he always inherits his X chromosome from his mother, who in this case does not have the altered gene.

However, any daughters the man has will become carriers of the mutated haemophilia gene and may pass it on to their children. 

If both parents have the changed gene

If a woman with the changed gene and a man with haemophilia have a baby, there's a:

• 1 in 4 chance of having an unaffected baby boy
• 1 in 4 chance of having a baby boy with haemophilia
• 1 in 4 chance of having a baby girl who's a carrier of haemophilia
• 1 in 4 chance of having a baby girl with haemophilia

This means it's possible for a female to have haemophilia, although it's very rare.

In some cases, a boy is born with haemophilia even though there's no family history of the condition.

In such cases, it's thought the gene change developed spontaneously in the boy's mother, grandmother or great-grandmother, but until then, a male member of the family had never inherited it.

Some studies have shown there's no known family history of haemophilia in up to 1 in 3 new cases.

Blood cells called platelets are very important for blood clotting. These cells have a sticky surface that allows them to clump together to stop the flow of blood.

Platelets also need clotting factors. These are proteins that form a "web" around the platelets, helping them to stay in place.

There are several different clotting factors in the blood. They're numbered using Roman numerals.

A child with haemophilia does not have enough of a certain clotting factor in their blood.

For example, a child with haemophilia A does not enough clotting factor VIII (8) in their blood. A child with haemophilia B does not have enough clotting factor IX (9) in their blood.