Males are far more likely to get hemophilia than females. Because the genes responsible for blood clotting factors are located on the X chromosome, and males have only one X chromosome, a single mutated gene is enough to cause the disorder. Hemophilia A, the more common type, occurs in about 1 in 4,000 male births, while hemophilia B occurs in roughly 1 in 20,000 male births.
Why Males Are Almost Always Affected
Hemophilia A and B are both caused by mutations in genes that tell the body how to make clotting proteins. These genes sit on the X chromosome. Since males have one X and one Y chromosome, they only get one copy of each clotting gene. If that single copy is faulty, there’s no backup, and the body produces too little clotting factor. The result is hemophilia.
Females have two X chromosomes, so even if one carries a hemophilia mutation, the other usually compensates by producing enough clotting factor to prevent serious problems. This is why females are typically called “carriers” rather than patients. But that label is misleading for a significant number of women.
Women Can Have Hemophilia Too
About one in four female carriers have clotting factor levels low enough to meet the current medical definition of hemophilia (below 40% of normal activity). And among carriers whose levels test above that cutoff, roughly another quarter still experience abnormal bleeding. These women are now classified as “symptomatic carriers,” a recognition that the old binary of “carrier versus patient” didn’t reflect reality.
Interestingly, low factor levels don’t always predict symptoms. A review of data from the American Thrombosis and Hemostasis Network found that nearly 60% of carriers with levels below 40% actually had normal bleeding scores for their age, a number that rises to 77% in childhood. So the relationship between clotting factor levels and real-world bleeding is more complicated than a simple threshold.
A girl can also inherit hemophilia outright if her father has hemophilia and her mother is a carrier, giving her two affected X chromosomes. This is rare but possible.
Family History Matters, but Not Always
About two-thirds of babies diagnosed with hemophilia have a known family history of the condition. The remaining one-third are born into families with no prior cases. These arise from spontaneous mutations, meaning the gene change happened for the first time in the child or the mother. In these families, the diagnosis typically comes only after an unusual bleeding episode rather than through early screening.
Severe hemophilia usually shows up in the first months of life, often during routine procedures or minor bumps. Mild and moderate forms can go undetected much longer, sometimes not surfacing until adolescence or adulthood, when a surgery, dental procedure, or injury triggers unexpected bleeding.
Race, Ethnicity, and Disparities
Hemophilia occurs across all racial and ethnic groups. Data from specialized treatment centers in the United States show slightly higher prevalence among White men (about 15 per 100,000) compared to Black or Hispanic men (about 12 per 100,000). But the more significant disparity involves complications: Black and Hispanic people with hemophilia consistently face a higher risk of developing inhibitors, which are antibodies that make standard clotting factor treatments less effective. They also tend to have worse physical functioning over time. Whether these gaps reflect biological differences, unequal access to care, or both remains an active question.
Hemophilia C and Specific Populations
There’s a third, rarer type called hemophilia C, caused by a deficiency in a different clotting factor (factor XI). Unlike types A and B, it isn’t linked to the X chromosome. It follows an autosomal recessive pattern, meaning both males and females are equally likely to be affected. It’s extremely rare in the general population, at roughly 1 in a million, but strikingly common among Ashkenazi Jews, where about 5% carry an abnormal copy of the gene. Elevated rates have also been documented in Iraqi Jews, French Basques, and a community in northeast England.
Acquired Hemophilia in Older Adults
Not all hemophilia is inherited. Acquired hemophilia A is a separate condition in which the immune system starts producing antibodies that attack the body’s own clotting factors. It follows a bimodal pattern: a smaller peak in adults under 30, mostly linked to pregnancy, and a much larger peak in older adults with a median age around 75.
In 50 to 80% of acquired cases, no clear underlying cause is found. When a trigger can be identified, autoimmune diseases like rheumatoid arthritis and lupus account for about 10% of cases. Malignancies and certain medications are also associated. One recent population-based study identified Alzheimer’s disease as a notable risk factor, though the biological connection is still being explored. Unlike inherited hemophilia, acquired hemophilia can strike anyone regardless of sex or family history, and it often appears suddenly with severe, unexplained bruising or bleeding in someone who never had clotting problems before.
Who Should Consider Genetic Testing
If you have a family history of hemophilia or unexplained bleeding problems, genetic testing and clotting factor assays can clarify your risk. Women with a known carrier status in their family can be tested before or during pregnancy to understand the likelihood of passing the gene to their children. Since about a third of cases arise without any family history, unexplained heavy bleeding after surgery, dental work, or childbirth is also worth investigating, particularly in women who may have been overlooked as potential carriers.