Hemophilia is a genetic disorder caused by missing or defective protein called factor, which is needed to form blood clots. Although it is passed down from parents to children, about 1/3 of cases are caused by a spontaneous mutation, a change in a gene. There are between 30,000 – 33,000 people with hemophilia in the US. All races and ethnic groups are affected. 

Hemophilia A, also called factor VIII (8) deficiency or classic hemophilia, is caused by missing or defective factor VIII (8) clotting protein. Hemophilia B, also called factor IX (9) deficiency or Christmas disease, is a caused by missing or defective factor IX (9), a clotting protein. Hemophilia A is four times as common as hemophilia B.  

The Genetics of Hemophilia

Hemophilia is an inheritable disease, meaning it is passed down from parents to children. The X and Y chromosomes are called sex chromosomes. The gene for hemophilia is carried on the X chromosome. Hemophilia is inherited in an X-linked recessive manner. Females inherit two X chromosomes, one from their mother and one from their father (XX). Males inherit an X chromosome from their mother and a Y chromosome from their father (XY). That means if a son inherits an X chromosome carrying hemophilia from his mother, he will have hemophilia. It also means that fathers cannot pass hemophilia on to their sons. 

Because daughters have two X chromosomes, even if they inherit the hemophilia gene from their mother, most likely they will inherit a healthy X chromosome from their father and not have hemophilia. A daughter who inherits an X chromosome that contains the gene for hemophilia is called a carrier. She can pass the gene on to her children. Many women who carry the hemophilia gene also have low factor expression, which can result in heavy menstrual bleeding, easy bruising, and joint bleeds. Some women who have the hemophilia gene have factor expression low enough to be diagnosed with hemophilia. 

For a female carrier, there are four possible outcomes for each pregnancy: 

  • A girl who is not a carrier 
  • A girl who is a carrier 
  • A boy without hemophilia 
  • A boy with hemophilia 

Symptoms & Severity

  • People with hemophilia bleed longer than other people. Bleeds can occur internally, into joints and muscles, or externally, from minor cuts, dental procedures, or injuries. How often a person bleeds, and the severity of those bleeds depends on how much factor is in a person’s plasma (the straw-colored fluid portion of blood). 

    Normal levels of factor FVIII or FIX in plasma range from 50% to 150%. Levels below 50%, or half of what is needed to form a clot, determine a person’s symptoms. 

    Mild hemophilia: Blood factor levels from 6% up to 49%. People with mild hemophilia typically experience bleeding only after serious injury, trauma or surgery. In many cases, mild hemophilia is not diagnosed until an injury, surgery, or tooth extraction results in prolonged bleeding. The first episode may not occur until adulthood. Women with mild hemophilia often experience menorrhagia, heavy menstrual periods, and can hemorrhage after childbirth. 

    Moderate hemophilia: Blood factor levels from 1% up to 5%. People with moderate hemophilia tend to have bleeding episodes after injuries. Bleeds that occur without obvious cause are called spontaneous bleeding episodes. 

    Severe hemophilia: Blood factor levels less than 1%. People with severe hemophilia experience bleeding following an injury and may have frequent spontaneous bleeding episodes, often into their joints and muscles.


Doctors will perform tests that evaluate how long it takes for the blood to clot to determine if someone has hemophilia. A clotting factor test, called an assay, will show the type and severity of hemophilia. 

Because of our understanding of hemophilia genetics, most people who have a family history of hemophilia will ask that their baby boys be tested soon after birth to see if they have hemophilia. If there is no family history of hemophilia, people often notice bleeding that takes longer to stop or lots of bruising. Many babies born with severe hemophilia are diagnosed if there is prolonged bleeding after circumcision. 

For girls, it often takes seeing worsening of symptoms for the diagnosis process to begin. Most girls are not tested for hemophilia before puberty. Heavy periods are a symptom of a bleeding disorder in women and girls. If there is a known family history, it is important to monitor for symptoms. It is recommended that women who have a family history of hemophilia get tested before getting pregnant, to help prevent complications during childbirth. 

The best place for patients with hemophilia to be diagnosed and treated is at one of the federally funded hemophilia treatment centers (HTCs) that are spread throughout the country. HTCs provide comprehensive care from skilled hematologists and other professional staff, including nurses, physical therapists, social workers and sometimes dentists, dieticians and other healthcare providers, including specialized labs for more accurate lab testing. 


The main medication to treat hemophilia is concentrated factor replacement product, called clotting factor or simply factor. There are two types of clotting factor: plasma-derived and recombinant. Plasma-derived factor is made from human plasma. Recombinant factor products are developed in a lab through the use of DNA technology. While plasma-derived factor products are still available, approximately 75% of the hemophilia community takes a recombinant product. 

These factor therapies are injected into a vein (called “infusion”) in the arm or hand, or through a port in the chest. The NBDF's Medical and Scientific Advisory Council (MASAC) encourages the use of recombinant clotting factor products over plasma-derived products because they are safer from blood-borne viruses and diseases. 

To maintain enough clotting factor in the bloodstream to prevent bleeds, patients with severe hemophilia are typically prescribed a regular treatment regimen, called prophylaxis – or prophy for short. This means a person will infuse their medication on a regular schedule – for example every day or every other day, depending on how long the factor lasts in the body.  

Treatments that use other ways of preventing bleeds are also available. These treatments are known as non-factor replacement therapies. To learn more about treatments for hemophilia click the button below. 

Living with Hemophilia

There's a lot to know about living with a bleeding disorder like hemophilia. Visit NBDF’s Steps for Living to explore resources, tools, tips and videos on living with hemophilia through all life stages. Organized by life stages, Steps for Living provides information on recognizing the signs of bleeds in children, help on navigating school issues, how to exercise safely, helping teens manage their bleeding disorder, information on workplace accommodations, and much more. There are downloadable checklists, toolkits, videos, and more.

Resource Links

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