The exact reasons why some babies have or develop birthmarks are not fully understood at present. It's not thought that vascular birthmarks are inherited. However, there is some evidence that birthmarks may possibly run in families.
I have a huge birthmark that is red that covers my right shoulder, and no one else in my family has EVER had one. It's really strange... I used to hate it, but now I LOVE it! Why do you ask? Does your baby have one? With my first baby my mom said she had dreams that my baby would have a birthmark on her face, and that is the first thing she looked at as she was coming out! lol! It really freaked her out. But I think they just make you extra unique!!