Isaiah 7:14

himself--since thou wilt not ask a sign, nay, rejectest the offer of one. you--for the sake of the house of believing "David" (God remembering His everlasting covenant with David), not for unbelieving Ahaz' sake. Behold--arresting attention to the extraordinary prophecy. virgin--from a root, "to lie hid," virgins being closely kept from men's gaze in their parents' custody in the East. The Hebrew, and the Septuagint here, and Greek (Mat 1:23), have the article, the virgin, some definite one known to the speaker and his hearers; primarily, the woman, then a virgin, about immediately to become the second wife, and bear a child, whose attainment of the age of discrimination (about three years) should be preceded by the deliverance of Judah from its two invaders; its fullest significancy is realized in "the woman" (Gen 3:15), whose seed should bruise the serpent's head and deliver captive man (Jer 31:22; Mic 5:3). Language is selected such as, while partially applicable to the immediate event, receives its fullest, most appropriate, and exhaustive accomplishment in Messianic events. The New Testament application of such prophecies is not a strained "accommodation"; rather the temporary fulfilment of an adaptation of the far-reaching prophecy to the present passing event, which foreshadows typically the great central end of prophecy, Jesus Christ (Rev 19:10).

— Jamieson-Fausset-Brown Commentary (public domain)