Port-wine stains are flat, reddish-purple birthmarks caused by an overgrowth of blood vessels in the skin. They are usually present at birth and can vary in size and shape. These marks are most commonly found on the face, neck, or limbs and can darken or thicken over time. While port-wine stains are harmless, they can sometimes be associated with Sturge-Weber syndrome, a neurological condition. Treatment options, such as laser therapy, can help reduce their appearance, but they typically do not disappear completely. Regular monitoring by a healthcare professional is recommended for those with larger or more prominent stains.