Bleeding gums are primarily caused by the accumulation of plaque at the gum line, leading to a condition called gingivitis, which is characterized by inflamed gums. If plaque is not removed, it can harden into tartar. This progression can cause increased bleeding and lead to a more severe form of gum and jaw bone disease called periodontitis.

Other causes include:
- Any bleeding disorders
- Brushing too hard
- Hormonal changes during pregnancy
- Ill-fitting dentures or other dental appliances
- Improper flossing
- Infection, which can be either in a tooth or the gum
- Leukemia, a type of blood cancer
- Scurvy, a vitamin C deficiency
- Use of blood thinners
- Vitamin K deficiency