A port-wine stain is a type of birthmark that appears as a flat, reddish-purple mark on the skin. It is caused by an overabundance of capillaries, which are small blood vessels, in a specific area. These stains can vary in size and may darken or thicken over time. They are most commonly found on the face, neck, or limbs. Port-wine stains are usually present at birth and do not fade away on their own. While they are harmless, some individuals may choose to seek treatment for cosmetic reasons. Options include laser therapy, which can help reduce the appearance of the stain.