God uses normal means to accomplish wonderous effects, and He does so for now, for the future, and for eternity.
Judah was in desperate straits. The strength and prosperity of King Uzziah had given way to the weakness and poverty of King Ahaz. Tilgath Pileser III, the ascendant ruler of Assyria, was expanding with a mighty army and his neighbors, Syria and Israel, had attacked Judah to force it to ally with them against Assyria. Judah had suffered a severe defeat, and at that moment, Ahaz was not thinking about something that was going to happen 730 years later. Probably, Isaiah wasn’t either. Therefore the child promised in Isaiah 7:14 was not, at least in Ahaz’ mind, the future Messiah. Isaiah had promised him a sign that God would deliver him and his nation from the combined might of Israel and Syria and the child was to be the sign. The sign was not that a young woman would bear a child; this is an ordinary part of human experience. Rather it was that the birth of this child would begin the countdown to destruction for Judah’s enemies. Specifically, the kings that Ahaz feared would be destroyed before the child reached preadolescence.
That prophecy was most likely fulfilled by the birth of Isaiah’s second son, Maher-shalal-hashbaz (MSH). According to Herbert M .Wolf, “almah” most reasonably refers to a young woman of marriageable age, who in Hebrew culture was expected to be a virgin. The event in 8:1-2 is likely the marriage ceremony between Isaiah and his new wife, and the child is conceived in verse 3. Isaiah’s prophetess was a virgin when she married but obviously not a virgin when she conceived. In verse 4, Isaiah clarified his prophecy; this time saying that before his son was even old enough to talk, Israel and Syria would be no more. Thus MSH was the immediate fulfillment of the prophecy in Isaiah 7:14.