Blood that does not clot properly

The short version

A missing protein, not thin blood

Clotting is a chain reaction. A dozen proteins in the blood, called clotting factors, have to act in sequence for a clot to form. A bleeding disorder is what happens when one of them is missing or does not work — usually because of an inherited genetic difference.

The result is not that a person bleeds faster. It is that bleeding does not stop on its own. The dangerous bleeds are usually the ones nobody can see: into a knee, an ankle, an elbow, or a muscle. Untreated, those cause severe pain and permanent joint damage, and a bleed into the brain or an internal organ can be fatal.

Bleeding disorders occur in every racial and ethnic group, and they affect both men and women.

Hemophilia

The best known, and not the most common

Hemophilia comes in two main forms, distinguished by which factor is missing:

Hemophilia A is a deficiency of factor VIII (factor eight). Hemophilia B is a deficiency of factor IX (factor nine), and is sometimes called Christmas disease, after the first patient described with it. Hemophilia A is three to four times as common.

Hemophilia occurs in roughly 1 in every 5,000 male births. It is usually inherited, but about 30% of people diagnosed have no family history at all — the genetic change happened spontaneously. Families are often blindsided.

Although it occurs mostly in men, women can and do have hemophilia, and women who carry the gene frequently have symptoms of their own.

Severity is a measurement, not an impression

Severity is defined by how much clotting factor is present in the blood, compared with normal levels. The word “mild” does a lot of damage here, because it describes a laboratory value rather than a life.

Inhibitors

Some people with hemophilia develop an inhibitor: the immune system treats the infused factor as a foreign body and produces antibodies that destroy it before it can work. Treatment stops working, and the alternatives are far more difficult and far more expensive. It is uncommon, and it is one of the main reasons access to the full range of products matters — when one product fails, a person needs the others.

Acquired hemophilia

Rarely, hemophilia appears in someone born without it, usually later in life, when the immune system begins attacking their own clotting factor. It is associated with autoimmune conditions such as rheumatoid arthritis and lupus, and with pregnancy and some cancers.

Von Willebrand disease

The common one almost nobody has heard of

Von Willebrand disease, or VWD, is caused by a missing or defective protein called von Willebrand factor, which helps platelets stick together and stabilizes factor VIII. It affects up to 1% of the population — making it by far the most common bleeding disorder, and far more common than hemophilia.

It occurs equally in men and women. Most cases are mild, and most people who have it do not know.

Like hemophilia, VWD can also be acquired later in life, through autoimmune disease, heart disease, some cancers, or as a side effect of certain medications.

Women and girls

Symptoms that get explained away

For decades, bleeding disorders were treated as a men’s disease, and women were described as “carriers” rather than patients. That has changed clinically, but it has not fully changed in practice.

A woman or girl with a bleeding disorder may present with heavy periods, prolonged bleeding after childbirth, iron deficiency treated without anyone asking why, or unexplained anemia. Each of these can be normalized — by her, by her family, or by a clinician who has never had reason to think about clotting.

The consequence has been measured, repeatedly, and the clearest way to see it is the comparison. A Lancet Haematology Commission published in July 2026 found that women wait an average of 14 to 16 years for a bleeding disorder diagnosis — compared with about two years for men.

A CDC study of women with von Willebrand disease found the same 16-year average, with a range running from no delay at all to thirty-nine years. A larger and more recent study found that more than 70% of adult women and half of adolescent girls later diagnosed with a bleeding disorder had experienced a delay: a median of ten years for the adults, four for the adolescents. Three separate bodies of research, arriving at the same place.

Sixteen years is not a wait for an appointment. It is the surgery nobody prepared for, the childbirth that went badly, the decade of iron supplements prescribed without anyone asking why. It is the reason there is now a bipartisan bill in Congress on exactly this problem, introduced by a Texas member of the US House.

The 14–16 years versus two years comparison: “Reducing mortality, improving outcomes, and establishing equity for women with classical haematological disease: a Lancet Haematology Commission,” The Lancet Haematology 2026;13(8):e555–e601, published 13 July 2026. The 16-year average: Kirtava A, Crudder S, Dilley A, Lally C, Evatt B, “Trends in clinical management of women with von Willebrand disease,” Haemophilia 2004;10:158–161 — a survey of 75 women enrolled at US hemophilia treatment centers. The 70% and median figures: Srivaths LV, Zhang QC, Byams VR et al., Haemophilia 2018;24:63–69. The last two are cited by the CDC in Bleeding Disorders in Women and Girls: State of the Science and CDC Collaborative Programs.

The rarer ones

Other factor deficiencies

Beyond hemophilia and VWD there are deficiencies of the other clotting factors — factors I, II, V, VII, X, XI, XII and XIII — and platelet function disorders. Each is rare, some extremely so.

For those families the practical problem is compounded: fewer specialists have seen the condition, fewer treatments are approved for it, and diagnosis takes longer. The case for comprehensive care at a treatment center is strongest precisely where the condition is rarest.