Your guide to all things hemophilia.

The deeper your understanding of hemophilia, the better prepared you’ll be to advocate for yourself and what you want out of life. Sanofi resources can help. Get information about what hemophilia is and how it’s inherited. You can also learn about how a blood clot forms, see common signs and symptoms of hemophilia, and more.

What is hemophilia?

Hemophilia is a rare, lifelong condition that prevents the blood from clotting due to missing clotting factor proteins. Hemophilia is caused by a genetic mutation on the X chromosome. In most cases, this mutation is inherited, but about 30% of the time it is caused by spontaneous mutation.

For people living with hemophilia, there are two common types:

Hemophilia A

Also called factor VIII (FVIII) deficiency, it is caused by missing or defective clotting factor VIII.

Hemophilia B

Also called factor IX (FIX) deficiency, it is caused by missing or defective clotting factor IX.

Learn what makes hemophilia B unique.

How hemophilia is inherited

If you have hemophilia in your family, knowing how the hemophilia gene is passed from generation to generation can help you make informed decisions for yourself and your family. Hemophilia mostly affects men who inherit the mutated X chromosome, but women who inherit this chromosome can be affected as well.

Please note: This chart is meant to help clarify how hemophilia is inherited by aligning gender identity to both sex assigned at birth and chromosomal makeup. If your gender does not match your sex assigned at birth or if your chromosomal makeup is not represented here, talk with your care team about your inheritance risk.

Male inheritance

Since males have one X and one Y chromosome, they will have hemophilia if they inherit an X chromosome carrying the hemophilia gene. Males inherit their X chromosome from their mother, so hemophilia is always passed down from a mother to a son unless there is a spontaneous mutation.

Female inheritance

Since females have two X chromosomes and receive one from each parent, fathers with hemophilia will pass the mutated gene on to a daughter 100% of the time. Mothers with the hemophilia gene are considered carriers and have a 50% chance of passing it on to a daughter.

Women who are carriers can also have hemophilia and show symptoms. Learn more about women and hemophilia.

How a blood clot forms

How the body forms a blood clot is a complex process, but it is important knowledge for anyone living with hemophilia. When a person has a bleeding injury, a process takes place called the clotting cascade. All steps in this process need to happen for stable clots to form. For people living with hemophilia, the lack of factor VIII or factor IX disrupts the clotting cascade and stable clots cannot form.

Let’s break down the steps at a high level:

After an injury occurs, platelets start forming a clot, but they need help sticking together to fully stop the bleeding. A series of proteins help form a mesh around the unstable platelet plug to form a stable clot. This is the start of the clotting cascade.

Once the clotting cascade is underway, factor VIII (missing or deficient in hemophilia A) becomes activated and then works together with activated factor IX (missing or deficient in hemophilia B) to activate factor X.

After factor VIII, IX, and X are activated, they work together to generate thrombin, which is necessary to form a stable clot.

Once thrombin is activated, it helps create the crucial fibrin mesh that spreads over the platelets, binding them together into the stable clot that stops the bleeding. If any factor is missing or not working, the clotting cascade won’t work and stable clots can’t form.

Visit our Levels Matter site to learn more about factor activity and the clotting cascade.

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Signs and symptoms

Recognizing the signs and symptoms is key to managing hemophilia. Symptoms can vary based on the levels of clotting factor in your blood and the severity of your condition. Common symptoms include:

Swelling, pain, or tightness in the joints, particularly the knees, elbows, or ankles
Bleeding after having shots, which could lead to muscle bleeds
Heavy menstrual periods
Bruising on the skin
Blood in the urine or stool
Low iron or have anemia
Bleeding in the mouth and gums that’s difficult to stop
Frequent and hard-to-stop nosebleeds

If you or someone you care for is experiencing one or more of these symptoms, contact your doctor. Bleeds that go unaddressed can potentially affect joint health. Learn more about joint health and bleed management.

Hemophilia voices

“We’ve come a long way, and I am ecstatic that there is more awareness and understanding of hemophilia than there was when I was a kid. I encourage those of you living with hemophilia to continue educating yourselves and working with your doctor to manage your hemophilia.”

Phil, living with hemophilia