Bleeding of the gums can be caused by many different factors such as diabetes, dry mouth syndrome, hormonal changes during pregnancy, and tobacco or substance abuse. The vast majority of the time, however, bleeding of the gums is caused by poor oral hygiene. Failure to brush and floss properly (or not enough) can lead to plaque buildup along and under the gum line. The accumulation of plaque results in inflammation of the gums, swelling, and bleeding. This disease is called gingivitis.