The word translated “virgin” (almah) refers to a young maiden who has never had sexual relations with a man. The wife of Isaiah who bore the son in fulfillment of the first aspect of the prophecy was a virgin until she conceived by Isaiah. However, according to Matthew 1:23–25, Mary, the mother of Jesus, was a virgin even when she conceived and gave birth to Jesus. The physical conception and birth of the son of Isaiah was a sign to Israel that God would deliver them from physical bondage to the Assyrians. But, the supernatural conception and birth of the Son of God was a sign to all of God’s people that He would deliver them from spiritual bondage to sin and death.
Geisler, N. L., and Howe, T. A. When critics ask: A Popular Handbook on Bible Difficulties, p. 267