hi..thanks for your query.
Nevus is the medical term for the birth marks.it can present at birth or appear shortly after birth, usually in the first month. They can occur anywhere on the skin.Birthmarks are caused by overgrowth of blood vessels, melanocytes,
smooth muscle, fat, fibroblasts, or keratinocytes i.e various structural components of the skin or its
connective tissue.So they can either be:
1)Pigmented birthmarks caused by excess skin pigment cells include
moles, café-au-lait spots, and Mongolian spots.
2) Vascular birthmarks are caused by blood vessels and include macular stains (salmon patches), hemangiomas, and Port-wine stains.
The vascular birth marks can change in appearance according to the amount of blood flowing into it, which might depend on activity of the child.
However, determination of the actual type of birth mark can only be done by inspecting it, so i would advise you to visit a
dermatologist for the same .It will also help you to know the further course of precautions/actions required,if any.
hope this helps..take care!