“Therefore, the Lord Himself will give you a sign: Behold, the virgin shall conceive and bear a Son and shall call His name Immanuel.” (Isaiah 7:14) This sign was given to the kingdom of Judah and her king, Ahaz. It was given to assure Judah and her king that the plot of the kingdom of Israel and Syria against Judah would not come to pass. “God would be with Judah” to protect her from this threat. The son in this context was Isaiah’s son, Maher-Shalal-Hash-Baz. We might say this was a partial fulfillment of the prophecy.
The prophecy was “filled-up” or completely fulfilled at the time of Christ Who was conceived in the virginal womb of His mother, Mary. Mary remained a virgin until the time of His birth! The name, Immanuel which means “God is with us” is more fully applied to Christ. Maher-Shalal-Hash-Baz was a “sign” that God was with us (Judah), but Jesus was/is Himself, God!