Hemophilia A is a hereditary bleeding disorder caused by a lack of blood clotting factor VIII.

Hemophilia A results from a deficiency (lack) of clotting factor VIII.

The disorder is caused by an inherited X-linked recessive trait, with the defective gene located on the X chromosome. Females have two copies of the X chromosome, so if the factor VIII gene on one chromosome doesn't work, the gene on the other chromosome can do the job of making enough factor VIII. Males, however, have only one X chromosome, so if the factor VIII gene on that chromosome is broken, they will have hemophilia A. Thus, most people with hemophilia A are male.

If a woman has a defective factor VIII gene, she is considered a carrier. This means the defective gene can be passed down to her children. In a woman who carries the defective gene, any of her male children will have a 50% chance of having hemophilia A, while any of her female children will have a 50% chance of being a carrier. All female children of men with hemophilia carry the defective gene. Genetic testing is available for concerned parents.

The severity of symptoms can vary. Severe forms become apparent early on. Bleeding is the main symptom of the disease and sometimes, though not always, occurs if an infant is circumcised. Additional bleeding problems are seen when the infant starts crawling and walking.

Mild cases may go unnoticed until later in life when they occur in response to surgery or trauma. Internal bleeding may happen anywhere, and bleeding into joints is common. Risk factors are a family history of bleeding and being male.

Rarely, adults can develop a bleeding disorder similar to hemophilia A. This may happen after giving birth (postpartum), in people with certain autoimmune diseases such as rheumatoid arthritis, in people with certain types of cancer (most commonly lymphomas and leukemias), and also for unknown reasons (called "idiopathic"). Although these situations are rare, they can be associated with serious, even life-threatening bleeding.