Causes of hemophilia A

What can lead to such a complex bleeding disorder? The answer may surprise you.

Understanding hemophilia A genetics

Our bodies produce a number of different clotting factors that work together in a sequence to produce a blood clot.

When the body can’t produce enough of the protein called factor VIII (factor 8), it can’t control bleeding. A mutation of a gene on the X chromosome causes the factor VIII deficiency known as hemophilia A.1

Hemophilia A is a sex-linked recessive genetic disorder. That means it can be passed from parents to children via a gene on the X chromosome.1

This gene provides instructions for making clotting factor VIII. When there is a mutation (change) in this gene, the body does not make the clotting factor properly.1

Girls inherit 2 X chromosomes, 1 from each parent. Boys inherit an X chromosome from their mother and a Y chromosome from their father. In a boy, the presence of the affected gene results in hemophilia symptoms because there is no second X chromosome that might have a properly working gene.1

Mother playing with her children.

How hemophilia A genetics work1

Diagram about how hemophilia A genetics work.

Father does not have hemophilia

Mother is a carrier of the hemophilia gene

50% chance sons will have hemophilia

50% chance daughters will be carrier of the hemophilia gene

Key for how hemophilia A genetics work  diagram.


Does not have hemophilia

Carrier of the hemophilia gene

Has hemophilia

Mutated gene

Normal gene

This diagram shows the inheritance pattern of a mother who is a hemophilia A carrier and a father who does not have hemophilia. Each pregnancy has a 25% chance of resulting in a male with hemophilia A, a 25% chance of resulting in an unaffected male, a 25% chance of an unaffected female, and a 25% chance of a female carrier.1 In rare instances, a female may also develop hemophilia A if both of her X chromosomes have the mutation, or 1 is affected and the other is missing or inactive.1

What is acquired hemophilia A?2

Hemophilia A can also be "acquired" with no family history of the disorder. Acquired hemophilia A occurs when a person's immune system attacks clotting factor VIII in the blood. It can be associated with various conditions, including pregnancy, autoimmune conditions, cancer, multiple sclerosis, and drug reactions. Acquired hemophilia A is very rare, occurring in about 1.5 cases per million persons.

Keep exploring

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What are the severity levels of hemophilia A?

Doctor discussing treatment options with a patient during an appointment.

Treatment options for hemophilia A

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How is hemophilia A diagnosed?

  1. Centers for Disease Control and Prevention. What is hemophilia? Accessed February 17, 2023.
  2. Haider MZ, Anwer F. Acquired Hemophilia. 2022 Sep 26. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2022 Jan.