Hemophilia is a genetic bleeding disorder in which your blood doesn’t clot properly because it’s missing one of the proteins needed to form a clot. About 12 out of every 100,000 males in the U.S. have hemophilia A, the most common form. People with hemophilia don’t bleed faster than anyone else, but they bleed longer, because their body can’t build a stable clot to stop it.
The condition ranges from mild to severe depending on how much clotting protein your body produces. With modern treatment, people with mild or moderate hemophilia have a life expectancy close to the general population, and even those with severe forms now live into their mid-70s on average.
Types of Hemophilia
There are two main types. Hemophilia A results from a deficiency of clotting factor VIII, and it accounts for roughly 80% of cases. Hemophilia B is caused by a deficiency of clotting factor IX. Both types produce similar symptoms and are managed with similar strategies, though the specific replacement proteins differ.
A third, rarer form sometimes called hemophilia C involves a different clotting factor (factor XI). It follows a different inheritance pattern and tends to cause milder bleeding, so it’s usually discussed separately from the classic forms.
How Hemophilia Is Inherited
The genes for clotting factors VIII and IX both sit on the X chromosome. Because males have only one X chromosome, a single defective copy is enough to cause hemophilia. Females have two X chromosomes, so they typically need defective copies on both to develop the full condition. This is why hemophilia overwhelmingly affects males.
Females who carry one altered copy are usually called carriers. In most carriers, the normal X chromosome picks up the slack and produces enough clotting factor for normal clotting. In some carriers, though, the body randomly shuts off the normal X chromosome in more than half of its cells (a natural process called X-inactivation that happens to skew unevenly). These women can end up with lower-than-expected clotting factor levels and may experience abnormal bleeding themselves.
Fathers with hemophilia cannot pass the condition to their sons, because sons inherit their father’s Y chromosome, not his X. They will, however, pass the affected X chromosome to all of their daughters, making every daughter a carrier. About one-third of hemophilia cases arise from a brand-new spontaneous mutation with no prior family history.
Severity Levels
Hemophilia severity is defined by how much clotting factor activity your blood has compared to normal:
- Mild: 5% to under 40% of normal activity. Bleeding problems typically show up only after surgery, dental work, or significant injury. Many people with mild hemophilia don’t learn they have it until adulthood.
- Moderate: 1% to 5% of normal activity. You may bleed excessively after minor injuries and occasionally experience spontaneous bleeding episodes.
- Severe: Less than 1% of normal activity. Spontaneous bleeding into joints and muscles can occur regularly, sometimes without any obvious trigger.
Symptoms to Recognize
The hallmark signs include unexplained and prolonged bleeding from cuts or injuries, large or deep bruises that appear easily, unusual bleeding after vaccinations or dental procedures, blood in your urine or stool, and frequent nosebleeds without a clear cause. In infants, unexplained irritability can sometimes be the first clue.
In severe hemophilia, the biggest concern is internal bleeding, particularly into joints. The knees, ankles, and elbows are the most commonly affected. Repeated bleeds into the same joint cause pain, swelling, and tightness, and over time they can permanently damage the cartilage and bone. This progressive joint damage, sometimes called hemophilic arthropathy, is the main long-term complication of the disease and the primary reason preventive treatment is so important.
How Hemophilia Is Diagnosed
Diagnosis usually starts with screening blood tests. The most informative is the activated partial thromboplastin time (APTT) test, which measures how long it takes your blood to form a clot using the specific clotting factors involved in hemophilia. In people with hemophilia A or B, this test shows a longer-than-normal clotting time. A second test called the prothrombin time (PT) measures a different set of clotting factors and typically comes back normal in hemophilia, which helps narrow down the diagnosis.
A complete blood count is also drawn. It’s usually normal in hemophilia unless someone has been bleeding heavily enough to lower their red blood cell count. The definitive step is a clotting factor assay, a blood test that directly measures the activity levels of factors VIII and IX. This confirms which type of hemophilia you have and how severe it is.
In about two-thirds of cases, the diagnosis is confirmed shortly after birth because the mother is a known carrier. In the remaining third, where a spontaneous mutation is responsible, the diagnosis often comes later, after unexplained bleeding leads to testing.
Treatment Approaches
The core treatment is replacing the missing clotting factor. This is done through an intravenous infusion of concentrated clotting factor, either derived from donated blood plasma or manufactured synthetically (recombinant). Treatment can be given on-demand, meaning only when a bleed occurs, or as routine prophylaxis, meaning regular infusions on a schedule to keep factor levels high enough to prevent bleeds from starting.
For people with severe hemophilia, long-term prophylaxis starting early in life is now the standard of care. The goal is to prevent the cycle of joint bleeds that leads to permanent damage. On-demand treatment remains an option for milder cases where spontaneous bleeds are infrequent. For hemophilia A specifically, a non-factor therapy is also available that mimics the function of the missing clotting factor without actually being factor VIII, offering an alternative for some patients.
Inhibitors: A Treatment Complication
One significant challenge is that the immune system can develop antibodies, called inhibitors, that attack the replacement clotting factor and make it ineffective. This happens in roughly 25% to 30% of people with severe hemophilia A and about 8% of those with severe hemophilia B, most often within the first 50 infusions. When inhibitors develop, treatment becomes more complex and may require alternative clotting agents or immune tolerance therapy to retrain the immune system to accept the replacement factor.
Gene Therapy
Gene therapy represents a newer treatment frontier. Instead of replacing the missing protein repeatedly, gene therapy delivers a working copy of the gene directly to liver cells, enabling the body to produce its own clotting factor. Two gene therapies have been approved in the U.S.: one for severe hemophilia A (approved in 2023) and one for hemophilia B (approved in 2022). Both are given as a single intravenous infusion. They’re currently approved only for adults and require screening to confirm the patient doesn’t already have antibodies against the viral delivery system used to carry the gene.
Life Expectancy and Outlook
The prognosis for hemophilia has improved dramatically. Data from the Netherlands, which has one of the most comprehensive hemophilia registries, shows that median life expectancy rose from 66 years in the 1970s and 1980s to 77 years in the 2001 to 2018 period, a gain of 11 years. During that same timeframe, life expectancy in the general male population rose only 4 years, from 79 to 83. People with moderate hemophilia reached a median of 80 years, and those with mild hemophilia reached 79, essentially matching the general population. Severe hemophilia still carries a gap, with a median life expectancy of 73 years, but that number continues to improve as prophylactic treatment and gene therapy become more widely available.