Bart Ehrman, December 2014, here:
Mark does not narrate an account of Jesus’ birth. Mark never says a word about Jesus’ mother being a virgin. Mark does not presuppose that Jesus had an unusual birth of any kind. And in Mark (you don’t find this story in Matthew and Luke!!), Jesus’ mother does not seem to know that he is a divinely born son of God. On the contrary, she thinks he has gone out of his mind. Mark not only lacks a virgin birth story; it seems to presuppose that they [sic] never could have been a virgin birth. Or Mary would understand who Jesus is. But she does not.
It’s no wonder that when Matthew and Luke took over so many of the stories of Mark, they decided, both of them, *not* to take over Mark 3:20-21. They had completely different view of Jesus’ mother and his birth.
