Are birthmarks hereditary??

my son was born with a big birthmark on his back ( a port-wine [pinkish red in color] that is in the shape of a Z, covering his whole right shoulder blade). i am pregnant with my second son, who is due in about a month, and i was wondering, "what are the chances of my second son having a birthmark also". i do not have any birthmarks (that i know of) but my husband has some discoloration on his neck/chest...although you can barely see it, even if you are looking for it. i am not sure if that is a birthmark or not. anyways, thanks in advance for your answers.