What I’d want to get across in a brief answer to this question is:
1) Greek parthenos is not necessarily a bad translation of Hebrew almah. The semantic ranges do overlap substantially, though (as Mark points out) parthenos tends more toward “virginity.”
2) For reasons that do not involved the translation of almah (in my opinion), Isaiah 7:14 is not intended as a Messianic prophecy.
3) I have heard people claim that Matthew was not asserting a virgin birth, but one has only to read the whole text to see that he clearly is doing so. Again, in my view, the correct translation in Matthew is “virgin” irrespective of one’s view of how almah should be translated in Isaiah 7:14. I would translate almah as young woman and also “is pregnant” rather than “shall conceive.”
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(In my notes here I’m speaking for myself, not attempting to summarize Goodacre’s arguments.)