Understanding Hemophilia -- the Basics

What Is Hemophilia?


Hemophilia refers to a group of inherited disorders that cause abnormal bleeding. The bleeding occurs because part of the blood -- the fluid part called plasma -- has too little of a protein that helps blood clot.

Symptoms of hemophilia range from increased bleeding after trauma, injury, or surgery to sudden bleeding with no apparent cause. There are two predominant types of hemophilia:

  • Hemophilia A -- also called classic hemophilia -- is most common and occurs in about 80% of people with hemophilia.
  • Hemophilia B -- also called Christmas disease -- is less common and occurs in about 20% of people with hemophilia.


Hemophilia A

Hemophilia A results from too little of a plasma protein called factor VIII, which helps blood clot. The greater the deficiency, the more severe the symptoms.

  • Mild hemophilia: 5% to 40% of the normal factor VIII level
  • Moderate hemophilia: 1% to 5% of the normal factor VIII level
  • Severe hemophilia: Less than 1% of the normal factor VIII level

Most people who have hemophilia A have moderate or severe symptoms.

Hemophilia B

Hemophilia B results from too little of a plasma protein called factor IX, which helps blood clot. As in hemophilia A, hemophilia B can be mild, moderate, or severe. The greater the deficiency, the more severe the symptoms.

Recognizing Hemophilia

If there is no family history of hemophilia, an infant would not be tested for the condition. However, if there is a family history of hemophilia, specific tests can be done from an umbilical cord blood sample to see if a newborn infant has hemophilia. In fact, if the family desires, such testing can be done before a child is born.

In moderate to severe hemophilia, most boys present with bleeding within their first 18 months of life. Symptoms in infants and children include:

  • Bleeding in the head during birth 
  • Swollen, bruised joints or muscles when learning to walk
  • Bleeding into joints spontaneously or from just learning to walk 
  • Bruising easily, from minor bumps 
  • Frequent nose bleeds or gum bleeds from teething or tooth brushing 
  • Bloody urine or stools


For mild hemophilia, you may not have any noticeable symptoms until you undergo a dental procedure, which may cause you to bleed heavily, or you may not have any unusual bleeding unless you are injured in an accident or have surgery.


Problems Caused by Hemophilia

Internal bleeding resulting from hemophilia may lead to several health problems, including:

  • Joint deformity: People with severe hemophilia may suffer spontaneous bleeding in the joints. People with less severe hemophilia may have bleeding in a joint if they fall or are injured. If joint bleeds are not promptly and adequately treated, joint deformity can occur. Bleeding in a joint can cause scarring in the joint. With repeated bleeds, the joint may lose mobility and result in a permanent limp. The knee, ankle, and elbow joints are most commonly affected. Bleeding into the muscles of the legs can also be severely disabling.
  • Blood in urine: Blood in the urine may occur on occasion, due to bleeding from the kidneys or bladder. If the bleeding is heavy, you can get blood clots that obstruct urine flow.
  • Problems breathing: Breathing problems can occur as a result of bleeding into the throat, nose bleeds, coughing or vomiting. If not promptly treated, bleeding in the throat can interfere with breathing so much that a patient may need to be put onto a ventilator until the bleeding stops and swelling goes down.
  • Brain damage: Bleeding in the brain may cause permanent brain damage and disability or even death. While usually caused by trauma, in very severe cases of hemophilia, these areas may bleed without known injury.





What Causes Hemophilia?

Hemophilia is a genetic disease caused by inheritance of a mutated gene on the mother's X chromosome. Though hemophilia can be from a new genetic mutation, it most often runs in families. While women may carry the mutated gene, it is their sons who inherit their single X chromosome from their mothers and are at a 50% risk for hemophilia. Daughters have two X chromosomes, one from each biologic parent, so even if she inherits the hemophilia mutation and becomes a carrier herself, the other X chromosome can provide normal clotting factors.

Men who have hemophilia do not pass the disease to their sons because boys inherit only a Y chromosome from their father. However, men do pass their X chromosome, and thus a defective gene, to each of their daughters, so each of their daughters is a carrier.


If the father has hemophilia and the mother is a carrier, there is a chance the daughter will have hemophilia.

About 70% of people who have hemophilia can trace hemophilia back through their family for multiple generations. In about 30% of newly diagnosed infants with hemophilia, no other family member is known to have had hemophilia. In these cases a mutation has occurred spontaneously in the factor VIII or IX gene.  The affected person (or new carrier) can subsequently transmit the defective gene to his or her offspring.

WebMD Medical Reference Reviewed by Minesh Khatri, MD on July 30, 2019



U.S. Library of Medicine Genetics Home Reference.

National Heart, Lung and Blood Institute.

National Hemophilia Organization.





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