Hemophilia A Causes

Most people with hemophilia A are born with the disorder. In some cases, it is acquired because of a spontaneous gene mutation.1

Hemophilia A is caused by a shortage of a protein in your blood called factor VIII. This protein is essential for normal blood clotting. When a blood vessel is injured, a blood clot forms to seal cuts and breaks at the site of injury to stop the bleeding. A person with hemophilia A is unable to form blood clots normally, leading to prolonged bleeding and easy bruising.2

The role of family history

The cause of hemophilia A can be explained by family history. Because hemophilia A is largely an inherited (congenital) disorder, the role of genetics and family history is important to understand.1

How genes cause hemophilia A:1,3

Example of Hemophilia A Inheritance Patterns
Hemophilia A Inheritance Patterns

Everyone is born with 2 sex chromosomes, one inherited from each parent. There are 2 types of sex chromosomes: X and Y. A female inherits one X chromosome from her mother and one X chromosome from her father (XX). A male inherits one X chromosome from his mother and one Y chromosome from his father (XY). The gene that causes hemophilia is located only on the X chromosome. A woman who gives birth to a child with hemophilia is often referred to as a “carrier.” Often, she will have other male relatives with hemophilia.

Sometimes, a baby will be born with hemophilia when there is no known family history. This means either that the gene has been “hidden” (that is, passed down through several generations of female carriers without affecting any male members of the family) or the change in the X chromosome is new (often called a “spontaneous” or “sudden” mutation).

If you are a female who is a carrier for hemophilia A, there are 4 possible outcomes for your child:

  1. A girl who is not a carrier
  2. A girl who is a carrier
  3. A boy without hemophilia
  4. A boy with hemophilia A

Although it is very rare, a female can be born with hemophilia if her father has hemophilia and her mother is a carrier.1,3

With each pregnancy, a woman who is a carrier has a 25% chance of having a son with hemophilia. Since the father’s X chromosome determines that a baby will be a girl, all daughters of a man with hemophilia will be carriers. Sons are determined by the father through the Y chromosome, so none of his sons will have hemophilia unless the mother is also a carrier.


  1. Hemophilia A. National Hemophilia Foundation website. http://www.hemophilia.org/Bleeding-Disorders/Types-of-Bleeding-Disorders/Hemophilia-A. Accessed October 14, 2014.
  2. Fast facts. National Hemophilia Foundation website. http://www.hemophilia.org/About-Us/Fast-Facts. Accessed October 14, 2014.
  3. What causes hemophilia? National Heart, Lung, and Blood Institute website. http://www.nhlbi.nih.gov/health/health-topics/topics/hemophilia/causes.html. Published July 31, 2013. Accessed November 11, 2014.