Rashi

the Lord, of His own, shall give you a sign He will give you a sign by Himself, against Your will. is with child This is actually the future, as we find concerning Manoah’s wife, that the angel said to her (Judges 13:3): “And you shall conceive and bear a son,” and it is written, “Behold, you are with child and shall bear a son.” the young woman My wife will conceive this year. This was the fourth year of Ahaz. and she shall call his name Divine inspiration will rest upon her. Immanuel [lit. God is with us. That is] to say that our Rock shall be with us, and this is the sign, for she is a young girl, and she never prophesied, yet in this instance, Divine inspiration shall rest upon her. This is what is stated below (8:3): “And I was intimate with the prophetess, etc.,” and we do not find a prophet’s wife called a prophetess unless she prophesied. Some interpret this as being said about Hezekiah, but it is impossible, because, when you count his years, you find that Hezekiah was born nine years before his father’s reign. And some interpret that this is the sign, that she was a young girl and incapable of giving birth.