What Causes It?

Hemophilia is caused by a defect in one of the genes that determines how the body makes certain blood-clotting factors. The genes are located on the X chromosomes, which determine if a baby is a boy or girl.
(Click Causes of Hemophilia to learn more about what causes the disorder and how it is inherited.)

Types of Hemophilia

There are two main types of the disorder: hemophilia A and hemophilia B. In hemophilia A, clotting factor 8 (VIII) is low or missing. Approximately 9 out of 10 people with hemophilia have type A. In hemophilia B, clotting factor 9 (IX) is low or missing.


The major signs and symptoms of the disease are bleeding and bruising.
Bleeding often occurs internally, in joints and muscles. If bleeding in the brain occurs, it is considered a medical emergency.
Hemophilia symptoms can be:
  • Mild
  • Moderate
  • Severe.
The severity of symptoms is determined by the amount of clotting factor in the blood. Most people (about 7 out of 10) with hemophilia A have the severe form of symptoms. People who do not have hemophilia have a clotting factor 8 activity of 100 percent. People with severe hemophilia A have a clotting factor 8 activity of less than 1 percent.
(Click Hemophilia Symptoms for more information.)

Making a Diagnosis

A doctor making a diagnosis will usually need to:
  • Review the patient's personal and family medical history
  • Perform a physical examination
  • Order blood tests.
The tests will indicate:
  • Whether there is a bleeding problem
  • If the bleeding problem is due to hemophilia or another cause
  • The severity of the disorder.
(Click Diagnosing Hemophilia for more information about how doctors diagnose this disease.)

Hemophilia Disease

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