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

How hemophilia A genetics work1

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


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

What are the severity levels of hemophilia A?


Treatment options for hemophilia A


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.