Medically birthmarks are still, for the most part, not very well understood. Many a study has looked at possible genetic links to try and explain why some form markings and others do not3. To date, most research teams and medical professionals alike agree that vascular birth marks are certainly not inherited. Why they occur, however, is yet to be determined.
Some medical professionals believe that strawberry mark birthmarks (also known as strawberry or capillary haemangiomas and nevus vascularis) possibly occur as a result of an over-accumulation of cells in the lining of blood vessels during foetal development. Others follow the thinking that tiny pieces of placenta may lodge inside the embryo during the initial stages of pregnancy, causing such marks to form. In a similar vein, a possible link to proteins produced by the placenta during pregnancy has also been looked at as a potential underlying cause of birthmark formation. As have potentially traumatic births associated with scarring that result in markings resembling birthmarks.
Other experts have pointed to the widening and narrowing function (dilation and constriction) of capillaries (thin blood vessels), which may experience some damage causing permanent widening and thus result in the development of, for example, a port wine stain marking. It is also believed that an accumulation of capillaries beneath the skin’s surface may have something to do with the formation of salmon coloured patches.
When a foetus develops, the cells that produce the central nervous system and the skin originate from the same cells. Therefore, abnormalities in the skin are sometimes associated with neurological/nervous system abnormalities. These are discussed further in this article.
Reference:
3. Golisano Children's Hospital - University of Rochester Medical Center Rochester. What is a Vascular Birthmark?: https://www.urmc.rochester.edu/childrens-hospital/craniofacial/vascular-birthmark.aspx [Accessed 29.08.2018]