Von Willebrand disease (VWD) is considered the most common inherited bleeding disorder – estimated to affect 1% of the population. Like hemophilia, it is caused by the deficiency of a specific blood clotting protein. This protein, called von Willebrand factor (VWF), helps platelets stick together to form a clot at the site of an injury. Low levels of VWF in the blood results in delayed or prolonged bleeding time. Depending on the severity of the disease, prolonged bleeding can occur spontaneously or following trauma or surgery. The lower the level of VWF factor in the bloodstream, the more at risk a person is for bleeding. Unlike hemophilia, VWD affects males and females equally. VWD is usually mild and often goes undiagnosed, but an estimated 3% of von Willebrand patients are considered severe. Some women who experience heavy menstrual bleeding may have VWD as the underlying cause.