Birthmarks are flat, usually pink or red, marks with irregular (not usually smooth or round) borders. Most birthmarks go away by themselves over time. They are usually gone by the time your child is 18 months of age. Birthmarks on the back of your child’s neck may stay for years, but they are harmless. If your child has a mark on their skin, it is a good idea for them to see a healthcare provider who knows a lot about birthmarks. Sometimes, a mark that looks like a birthmark is something different. For example, some marks could be port wine stains or hemangiomas. Those are two conditions that need more care. Read more.