Let me make a couple of preliminary observations:
(1) This is a debated passage. (aren’t they all).
(2) The NT in Matthew clearly sees Isa. 7:14 as a prophecy of Christ.
a. In fact, the Greek translation of the OT clearly translated the term “virgin” in Isaiah.
b. I believe there was some Jewish expectation that this was messianic (although, if I remember correctly, Jews after the first century changed their view in a polemic against Christianity).
c. Either way, Isaiah is clear with Isaiah 7:14 and Jesus’ birth. The latter is a fulfillment of the former.
d. Isaiah also makes some clear connections to the significance of “God with us.” But in Numbers 14:9 the phrase “The Lord is with us” does not speak of the incarnation but God’s protection of His people. So Isaiah could just mean “protection”)—while Matthew clearly means it in a unique significance similar to John 1:14 “and the Word became flesh”.
(3) The context of Isaiah is a little rough to sort through. Here are probably the major issues.
a. The sign is supposed to be for the day of Isaiah. Before the child knows to reject right and wrong the land of the two kings will be laid to waste. This attack is something that happens in Isaiah’s day. This makes it hard to see how they would recognize the sign if it wasn’t until apprx. 4 BC that Christ was born.
b. There is a little debate about the Hebrew term for virgin. Does it mean a woman who was a young woman? Does it mean a virgin?
i. And does Isaiah actually mean that the women will be a virgin when the baby is born. (i.e. she could refer to a girl who is a virgin at the time of speaking without presupposing she will be a virgin when she gives birth—like Mary was).
ii. The problem is compounded by the fact that in Ugaritic the phrase “a virgin will give birth” was a way of describing a young maiden who would be engaged, be married and have a child.
c. The role of Maher-Shalal-Hash-Baz. Who is he (in ch. 8)? What is his role in relationship to 7:13-17? Some scholars think that he might be a fulfillment of Isaiah 7. Interestingly Isa. 8:4 might be hinting at this.
d. Some suggest that the child might be Ahaz’s son—Hezekiah, another Davidic King (of which Christ is also from the line of).
Here would be some of my conclusions:
(1) The focus of the early chapters in Isaiah at parts does focus on the Messiah.
a. Isaiah 9:1-6 clearly prophesies about the Messiah.
b. Isaiah 11 looks for the triumph of son from David’s line.
(2) The Hebrew word translated “virgin” does mean virgin. So whoever Isaiah was talking about she was a virgin at the point of his statement (or yet to be born).