@Howie The almah/parthenos debate doesn't seem to be all that relevant here. Allow me to explain.
First of all, it is widely recognized that Hebrew didn't have a word like our English "virgin" or the Latin term
virgo intacta, which referred in a precise sense to a person's sexual status. Even the Greek word
parthenos appears to refer principally to a young woman of marriageable age who has not yet been married, with the connotation of virginity but not with that technical meaning. Moreover, it's not even clear that the translation of
almah into the LXX's
parthenos in Isaiah 7:14 happened prior to Matthew's use of it.
That being said, the real problem here is that even if the author of the Hebrew Isaiah intended to refer specifically to virginity---which, again, is very probably not the case---the passage in Isa 7:14 is not evidently a prophecy about a virginal conception, but only speaks of a woman who (presumably) is now a virgin, but
will conceive in the future through natural means.
Larry Chouinard (Kentucky Christian College), explains (
Matthew, The College Press NIV Commentary (1997), pp51-2):
While Matthew clearly teaches the virginal conception of Jesus, his use of Isaiah 7:14 has been the subject of extensive discussion. The apparent absence of a pre-Christian Jewish messianic interpretation of Isaiah 7:14 has led some to suggest that Matthew either misunderstood Isaiah or falsified the text to fit his Christian presuppositions. After all, as many have observed, one may understand the Isaiah prediction to refer to a "young girl," presently a virgin, who would at some later time conceive and give birth, without any miraculous overtones. Neither is the linguistic evidence conclusively in support of a strictly messianic prediction of Jesus' virginal conception. The LXX parthenos translates the Hebrew term almah which is not a technical term demanding the translation "virgin" (cf. Prov 30:19); although contextually its usage in the OT seems to accent both a young girl's marriageable age as well as her virginity. It may be argued that the LXX's rendering of almah with parthenos conclusively demonstrates that at least some Jewish translators understood almah as referring to a "virgin" and not merely a "young woman." Although virginity is ordinarily implied by parthenos some have pointed to Gen 34:3 as an exception. Even if Isaiah had in mind a conception by a virgin, it is still possible to understand the fulfillment as a purely natural conception of a young woman who at the time of Isaiah's prediction to Ahaz was a virgin. Therefore, notwithstanding the extensive linguistic discussions that the term "virgin" has stirred, a more convincing line of argument detailing Matthew's use of Isaiah lies elsewhere.