Two things...
1. The virgin birth of Jesus comes from the prophecy in Isaiah 7:14. The problem is that the Hebrew word used means "maiden" and not "virgin," but the English translations insist on using virgin. Ironically, the U.S. Council of Catholic Bishops recently agreed that the correct term in Isaiah 7:14 does indeed mean maiden, not virgin. There is no virgin prophecy.
2. In the NT, you find where Mary is pregnant and Joseph is not exactly happy about it. An angel comes and explains the situation to him, at which point he is all cool with it. The Holy Spirit caused Mary's pregnancy, and since Jewish girls were assumed to be virgins until marriage...you get the idea. You don't need an OT prophecy in order for her to be a virgin. She already fulfilled the Isaiah 7:14 prophecy by being a maiden, that just happened to be a virgin, or so it is implied.
However, instead of saying "The Virgin Mary," I just say "Mary, Mother of Jesus" and be done with it.