This is how the birth of Jesus Christ came about […] All this took place to fulfill what YHWH had said through the prophet: “Behold! The virgin will be with child and will give birth to a son, and they will call Him Immanuel”(which means, “God with us”).
[Matthew 1]
Isaiah 7 tells the saga of Judah, under King Ahaz, being threatened in war and afraid of their enemies’ victory over them. God tells them not to be afraid and encourages them saying, “It shall not stand, neither shall it happen.”
He bids them to “ask a sign of Yehowah your God; ask it either in the depth, or in the height above.” The sign would give them confirmation that “It shall not stand, neither shall it happen.” But King Ahaz was also afraid to tempt God. God said (paraphrased) ‘I’ll give you a sign anyway.’ This famous passage is God’s description of the sign that those that would war against them would not win:
“Therefore the Lord himself will give you a sign: Behold, the young woman has conceived, will have a son, and name him Immanuel. He will eat butter and honey when he knows to refuse the evil, and choose the good. For before the child knows to refuse the evil, and choose the good, the land whose two kings you abhor shall be forsaken.”
The word often mistranslated “virgin” from the Hebrew “almah” has nothing to do with sexuality/virginity; rather it denotes an age-range of a female — a “young woman.” In addition, properly translated, it does not say “will conceive” but rather “is pregnant” or “has conceived”.
The sign was that she was already pregnant and by the time she delivered her baby, and before the baby is weaned from milk and knows right from wrong (approximately six months from birth), the kingdoms that they feared would be abandoned/non-existent.
While it is unknown who the young woman was, it makes sense that King Ahaz knew her and could be encouraged as he witnessed the prophecy unfold through the life of the child.