A second Old Testament building block that would point ultimately to the virgin birth can be found in Isaiah 7. It is a well-known prophetic statement, especially well known because it is lifted out of its Old Testament context and quoted by Matthew in the first chapter of his book, which gives the story of the birth of Jesus.
There are two contexts to this Isaiah passage. One is that of the immediate time of Isaiah, when the people of Judah, centered in the capital city of Jerusalem, are under attack. King Ahaz doesn't know what to do. Isaiah tells him to keep calm, not be afraid. Ahaz is astounded at this. Terrible things are going on, and he is about to be ruined. Isaiah tells him that God will give him a sign, "'Therefore the Lord himself will give you a sign. Look, the young woman is with child and shall bear a son, and shall name him Immanuel'" (Isaiah 7:14). He is telling Ahaz not to worry, matters will work out.
Matthew, many years later, takes this particular story totally out of its context and applies it to the virgin birth and sees it as a prophetic utterance, a prediction of what is going to happen. Those of us who believe that the Bible is inspired by the Holy Spirit, that Matthew is inspired and Isaiah is inspired, have no difficulty in saying that the Holy Spirit wants to inspire Matthew to see that prophetic meaning in Isaiah.
There are the skeptics who want to refute this. They say that the Hebrew word translated "virgin," in some translations of the Bible, is the word alma. That word does not necessarily mean a virgin. It can simply mean a young woman. The NRSV Bible we are using translates it "young woman." Immanuel means "God with us." Some may say that the sign simply was that a young woman was going to have a baby, and she would call his name Immanuel, and that would be a wonderful encouragement and reminder to the people that God is with us.
Yes, we can come to this Old Testament passage and see it in its two contexts. We use here a technical theological phrase, sensus plenior, for what is referred to as the "fuller sense" of Scripture. It is quite possible that what Isaiah said to King Ahaz had two prophetic meanings. One was an immediate fulfillment of a child being born who was called Immanuel, and then the fuller sense of the greatest sign to the house of David that the triumph would come through Immanuel, born of a virgin. This is the prediction of Jesus Christ, the ultimate God with us. There is that word for the present, and an additional word for the future.
The very fact that Matthew, under the inspiration of the Holy Spirit, goes back hundreds of years and quotes Isaiah 7, is confirmation to me that this is an Old Testament prophetic building block, predicting the future birth of Jesus.