Hemophilia is a genetic bleeding disorder in which the blood doesn’t clot properly because it’s missing or low on a specific clotting protein. It affects roughly 1 in 5,600 live male births in the United States, with about 400 boys born with the most common form each year. The condition ranges from mild to severe, and modern treatment has brought life expectancy close to that of the general population.
How Blood Clotting Goes Wrong
When you cut yourself, your body launches a chain reaction involving more than a dozen clotting proteins (called “factors”) that work together to form a stable clot and stop bleeding. In hemophilia, one link in that chain is weak or missing entirely, so clots form too slowly or not at all.
There are two main types. Hemophilia A, sometimes called classic hemophilia, involves a shortage of clotting factor VIII. It’s four times more common than hemophilia B, also known as Christmas disease, which involves a shortage of clotting factor IX. Both types produce similar symptoms and are managed in similar ways, but the specific missing protein determines which treatment a person needs.
Severity Levels
Hemophilia isn’t one-size-fits-all. Doctors classify it by how much functional clotting factor is circulating in the blood, using categories established by the International Society on Thrombosis and Hemostasis:
- Mild: Factor levels between 5% and 40% of normal. Bleeding problems typically show up only after surgery, dental work, or significant injury.
- Moderate: Factor levels between 1% and 5%. Excessive bleeding can follow relatively minor bumps or trauma.
- Severe: Factor levels below 1%. Bleeding can start spontaneously, without any obvious trigger.
Most of the serious complications people associate with hemophilia occur in the severe category, which accounts for roughly half of all hemophilia A cases.
Why It Runs in Families
The genes for both clotting factor VIII and factor IX sit on the X chromosome. Since males have only one X chromosome (inherited from their mother), a single faulty copy of either gene is enough to cause hemophilia. Females have two X chromosomes, so a normal copy on one usually compensates for a faulty copy on the other.
This inheritance pattern means fathers cannot pass hemophilia to their sons (fathers contribute a Y chromosome, not an X). A mother who carries one altered copy has a 50% chance of passing it to each child. Her sons who inherit it will have hemophilia; her daughters who inherit it will typically be carriers.
That said, some female carriers do experience bleeding symptoms. In roughly half of their cells, the X chromosome carrying the normal gene gets randomly switched off through a natural process. If that inactivation is uneven, a carrier can end up with meaningfully lower clotting factor levels and face real bleeding risks, not just the theoretical ones.
What Bleeding Looks Like
Hemophilia doesn’t usually mean visible, dramatic cuts that won’t stop. The bigger concern is internal bleeding, especially into joints and muscles. Joint bleeding (hemarthrosis) is the most common musculoskeletal problem in hemophilia. In severe cases, 75% to 90% of patients experience it, with the first episode typically happening between ages 2 and 3. The knee is the joint most often affected, though elbows, ankles, and shoulders are common targets too.
A joint bleed feels like sudden warmth, swelling, and tightness. Older children and adults often notice a tingling or stiffness before the pain sets in. In infants, the signs are subtler: fussiness and reluctance to use one arm or leg. Repeated bleeding into the same joint causes lasting damage. Blood inside the joint triggers inflammation that gradually destroys cartilage and thickens the joint lining. Over time, this creates what specialists call a “target joint,” one that bleeds more easily and develops chronic, disabling arthritis.
Other common bleeding problems include large, deep bruises, prolonged bleeding after dental procedures or surgery, and in rare cases, bleeding inside the skull, which is a medical emergency.
How Hemophilia Is Diagnosed
Diagnosis starts with blood tests that measure how long it takes blood to clot. A test called a partial thromboplastin time (PTT) is typically prolonged in hemophilia, while other clotting tests come back normal. That pattern points doctors toward a specific factor assay, a blood test that measures the exact level of factor VIII or IX. The result, expressed as a percentage of normal, determines both the type and severity of the condition.
In families with a known history, genetic testing can identify carriers and diagnose hemophilia before symptoms appear, sometimes even before birth.
Treatment: Replacing What’s Missing
The core of hemophilia treatment is straightforward in concept: put back the clotting factor the body can’t make on its own. Factor replacement therapy delivers concentrated factor VIII or IX into the bloodstream through an intravenous infusion. This can be done “on demand” to stop a bleed that’s already started, or on a regular schedule (prophylaxis) to prevent bleeds from happening in the first place. Prophylaxis is the standard of care for severe hemophilia and has dramatically reduced joint damage compared to older treatment approaches.
For hemophilia A specifically, a newer option works differently. Rather than replacing factor VIII directly, a bispecific antibody given as a shot under the skin mimics what factor VIII does by bridging two other clotting proteins. This approach has significantly reduced bleeding frequency and is especially valuable for patients whose immune systems have developed a complication called inhibitors.
The Inhibitor Problem
Between 20% and 35% of people with hemophilia A develop inhibitors, antibodies that recognize the infused clotting factor as foreign and neutralize it. This makes standard factor replacement far less effective and bleeding episodes much harder to control. Patients with inhibitors need alternative treatments called bypassing agents, which activate the clotting cascade through a different route. Managing inhibitors is one of the biggest challenges in hemophilia care.
Gene Therapy
In June 2023, the FDA approved the first gene therapy for adults with severe hemophilia A. The treatment delivers a working copy of the gene for factor VIII using a modified virus as a carrier, with the goal of enabling the body to produce its own clotting factor. It’s a single intravenous infusion, and eligible patients must not have pre-existing antibodies to the viral carrier used in the therapy.
Gene therapy represents a fundamental shift from managing hemophilia to potentially correcting it at the source. Results so far show meaningful increases in factor levels, though durability over decades is still being tracked.
Life Expectancy Today
Before modern treatment existed, severe hemophilia often meant a dramatically shortened life. That picture has changed. A 2022 study from France found that people with mild or moderate hemophilia actually had slightly higher life expectancy than the general population, likely because of the frequent, comprehensive medical follow-up they receive. Those with severe hemophilia had life expectancy broadly comparable to the general population at age 40, though a gap of up to 10 years remains depending on severity and complications.
The improvement reflects decades of advances: better clotting factor concentrates, widespread use of prophylaxis, newer non-factor therapies, and now gene therapy. For a child born today with hemophilia, the realistic expectation is a full, active life with careful management rather than the disability and early death that defined the condition a generation ago.