Hemophilia Types and Severity Levels

If you or your child has just been told "hemophilia A, mild" or "hemophilia B, severe," you probably have questions about what those words mean. Dr. Joe Moleski, Hemo-Health's physician, lives with hemophilia himself. At Hemo-Health, we believe the label matters less than knowing what it means for your own body.

This guide explains the types of hemophilia, how doctors grade severity, and why "mild" never means "minor."

What is the difference between hemophilia A and B?

Hemophilia is a bleeding disorder. Your blood has proteins called clotting factors that help stop bleeding. In hemophilia, one of them is missing or low.

• Hemophilia A is caused by a lack or low level of factor VIII (8) [1].
• Hemophilia B is caused by a lack or low level of factor IX (9) [1]. It is also called factor IX deficiency or Christmas disease [5].

Hemophilia A is about three to four times as common as hemophilia B [1]. Inherited hemophilia occurs in about 1 of every 5,000 male births [1].

In both types, how much a person bleeds generally matches how low their factor level is [3]. That is why the factor level, not the letter, decides the severity. For the basics of how hemophilia works and is passed down, see What is Hemophilia?

What about hemophilia C and other types?

Hemophilia C is another name for factor XI (11) deficiency [6]. It is different from A and B in a few ways:

• Men and women are affected equally [6].
• It is rare in the general population (about 1 in 100,000) but much more common among Ashkenazi Jewish people [6].
• Factor XI levels do not always match bleeding symptoms. Some people with low levels bleed less than people with higher levels [6].
• Bleeding most often shows up after a tooth is pulled, surgery or an injury. Joint and muscle bleeds are uncommon [6].

Acquired hemophilia is a separate, rare condition. It develops later in life, when the immune system makes antibodies against the body's own factor VIII. It can happen in people with no past bleeding problems [1][9].

What do mild, moderate and severe mean?

Severity depends on how much clotting factor is in your blood. The lower the level, the more likely bleeding is [1]. Normal factor VIII or IX levels are between 50% and 150% [4].

The World Federation of Hemophilia (WFH) uses these levels [3]:

Severity	Factor level	Typical bleeding pattern
Severe	Below 1% of normal (under 0.01 IU/mL)	Bleeding into joints or muscles with no clear cause
Moderate	1% to 5% of normal (0.01 to 0.05 IU/mL)	Occasional bleeding with no clear cause; long bleeding after minor injury or surgery
Mild	5% to under 40% of normal (0.05 to 0.40 IU/mL)	Severe bleeding with major injury or surgery; bleeding with no clear cause is rare

Source: WFH Guidelines for the Management of Hemophilia, 3rd edition (2020), Table 2-1 [3].

Sources differ a little at the edges. The National Bleeding Disorders Foundation (NBDF) defines mild as 6% to 49% [4][5]. If your level is near a cut-off, ask your care team which definition your lab report uses.

A person with hemophilia will usually have the same severity over their lifetime [8]. Even so, it is worth reviewing your care plan with your care team over time.

What are the symptoms of mild hemophilia?

People with mild hemophilia may not have abnormal or long bleeding until they have a serious injury or surgery [3]. Diagnosis often comes later. CDC data show the median age at diagnosis is 36 months for mild hemophilia, compared with 8 months for moderate and 1 month for severe [2].

Signs that can point to hemophilia include [1][4]:

• bleeding that is hard to stop after losing or pulling a tooth
• frequent nosebleeds that are hard to stop
• easy bruising, or lumps of blood under the skin
• bleeding after shots or vaccinations
• blood in the urine or stool
• heavy periods in women and girls

If any of these sound familiar, read Recognizing Bleeding Symptoms.

Can mild hemophilia be serious?

Yes. "Mild" describes the factor level, not the risk of a single bleed. In mild hemophilia, severe bleeding can happen with major injury or surgery [3].

That is the danger. Someone with mild hemophilia may not know they have it, or may not mention it before a procedure. A surgeon or dentist who does not know about the bleeding disorder cannot plan for it.

Inhibitors are another reason to stay in regular care. These are antibodies that stop treatment from working well. About 1 in 5 people with hemophilia A and about 3 in 100 people with hemophilia B develop one [2]. Treatment depends on the type and severity of your hemophilia and on whether you have an inhibitor, so talk with your care team before any surgery or dental work.

Can women and girls have mild hemophilia?

Yes. Hemophilia is much rarer in females, but females can have it, and some carriers have symptoms [1]. Bleeding in carriers is often overlooked and underdiagnosed, and joint bleeds can go unrecognized [3].

In 2021, the International Society on Thrombosis and Haemostasis (ISTH) set out clearer names for women and girls [7]:

• Mild hemophilia: factor level above 0.05 and below 0.40 IU/mL (above 5% and below 40%)
• Moderate hemophilia: 0.01 to 0.05 IU/mL
• Severe hemophilia: below 0.01 IU/mL
• Symptomatic carrier: 0.40 IU/mL or higher, with bleeding
• Asymptomatic carrier: 0.40 IU/mL or higher, without bleeding

Factor levels in females are harder to predict and can differ from other family members [8]. A woman cannot assume her level matches her father's or brother's. A factor level test is the way to know. When you contact us, we'll explain how factor-level testing works for your situation.

For more, read Women as Hemophilia Carriers.

Frequently asked questions

Is hemophilia A worse than hemophilia B?

Not by type alone. In both, bleeding generally matches the factor level [3]. A person with severe hemophilia B can bleed more than a person with mild hemophilia A.

What factor level is mild hemophilia?

WFH defines mild as 5% to under 40% of normal [3]. NBDF uses 6% to 49% [4]. Your care team can explain your own result.

Does hemophilia severity change over time?

It usually stays the same over a lifetime [8]. In women and girls, factor levels are less predictable, and factor VIII levels can change with pregnancy, hormone treatment and age [8].

Should you tell your dentist or surgeon you have mild hemophilia?

Yes. Bleeding problems in mild hemophilia often appear with surgery or injury [3]. Tell every dentist and surgeon well ahead of time, and involve your hemophilia care team in planning.

Can a second opinion help you understand your diagnosis?

It can help if your label does not match your bleeding, or if you were never fully tested. See Getting a Second Opinion.

Become a patient

If you have questions about your type, severity or test results, our team can see you by secure telehealth video visit. We work alongside your hemophilia treatment center (HTC) and do not replace it. Availability depends on your state and licensure; see States We Serve. Learn more about Hemophilia Care, then request a visit with the intake form or call 636-628-6604. Hemo-Health is not an emergency service.

In an emergency: A bleed in the head, throat or abdomen, or any serious injury, is an emergency: call 911 and tell them you have hemophilia. Then contact your hemophilia treatment center (HTC).

Sources

1. Centers for Disease Control and Prevention (CDC). About Hemophilia. https://www.cdc.gov/hemophilia/about/index.html. Accessed 2026-10-21.
2. Centers for Disease Control and Prevention (CDC). Data and Statistics on Hemophilia. https://www.cdc.gov/hemophilia/data-research/index.html. Accessed 2026-10-21.
3. World Federation of Hemophilia (WFH). WFH Guidelines for the Management of Hemophilia, 3rd edition (Srivastava A, et al. Haemophilia 2020;26(Suppl 6):1–158), Chapter 2 and Table 2-1. https://www1.wfh.org/publications/files/pdf-1863.pdf. Accessed 2026-10-21.
4. National Bleeding Disorders Foundation (NBDF). Hemophilia Fact Sheet (March 2024). https://www.bleeding.org/sites/default/files/document/files/Hemophilia-Fact-Sheet.pdf. Accessed 2026-10-21.
5. National Bleeding Disorders Foundation (NBDF). Hemophilia B. https://www.bleeding.org/bleeding-disorders-a-z/types/hemophilia-b. Accessed 2026-10-21.
6. National Bleeding Disorders Foundation (NBDF). Factor XI Deficiency. https://bleeding.org/bleeding-disorders-a-z/types/other-factor-deficiencies/factor-xi. Accessed 2026-10-21.
7. International Society on Thrombosis and Haemostasis (ISTH). van Galen KPM, et al. A new hemophilia carrier nomenclature to define hemophilia in women and girls: Communication from the SSC of the ISTH. J Thromb Haemost 2021;19(8):1883–1887. https://doi.org/10.1111/jth.15397. Accessed 2026-10-21.
8. Haemophilia Foundation Australia. Haemophilia severity. https://www.haemophilia.org.au/bleeding-disorders/haemophilia/haemophilia-severity/. Accessed 2026-10-21.
9. GeroScience (peer-reviewed). Lehoczki A, et al. Acquired hemophilia A as a disease of the elderly: a comprehensive review of epidemiology, pathogenesis, and novel therapy. GeroScience 2025;47(1):503–514. https://doi.org/10.1007/s11357-024-01317-7. Accessed 2026-10-21.