It is, I believe Christian doctrine that Jesus was born of a virgin. But in Matthew the biblical text maintains not merely that Jesus was born of a virgin, but was prophesied to be so born.
Matthew 1: 22-23, NIV
22 All this took place to fulfill what the Lord had said through the prophet: 23 “The virgin will conceive and give birth to a son, and they will call him Immanuel”[g] (which means “God with us”).
This is, of course, a quote from Isaiah 7: 14. There is some controversy as to whether the Hebrew word in the prophecy really means "virgin" or "young woman." But that is not my worry about the use of this. But that is not my concern. My concern is that it looks as is Matthew has ripped the Isaiah prophecy out of its context. The context is this: Pekah the King of Israel, and Rezin the king of Syria are threatening to attack Ahaz king of Judah. Ahaz is scared, and wants to go get protection from the Assyrians. Isaiah is telling Ahaz to trust God, not Nineveh, for protection against Pekah and Rezin, and tells him to look for a sign from God. The sign is supposed to be that a virgin or young woman (however you translate it) will conceive and bear a son, showing Ahaz that God is with us (and that he doesn't have to go do business with the stinking Assyrians (not nice people, by the way) to maintain the security of Judah.
But if that's the sign Isaiah is talking about, then the birth of Jesus, which takes place several centuries after Ahaz is dead, doesn't do the job. Ahaz needs a sign NOW that God is with us. So how is the Isaiah verse a prophecy of Jesus?