Actually, it's not immediately clear from Isaiah 7:14 that a virgin birth is predicted. The Hebrew is almah, which refers to a young woman without regard to her sexual experience. It can refer to young, unmarried virgins, but it does not have to.
Now, the LXX rendering of Isaiah 7:14 and the quotation thereof found in Matthew use the Greek word parfenoV (parthenos), which can only mean a virgin. But it is not immediately clear from the Hebrew text that that is the meaning of Isaiah 7:14. Indeed, the conception of Maher-shalal-hash-baz (which was the first fulfillment of the prophecy) was completed in the very ordinary fashion.
Once again, this is another matter where Christian interpretation of the Old Testament is shown to be dependant upon the New Testament's guidence.
Yes, it is immediately clear. I've been studying hebrew for 31 years, have taught it in seminary. You?
Dan
Yup. If you want more, read Irenaeus' Proof of Apostolic Preaching. Lots and lots of OT quotes that are read through the Christian proclamation. Justin's Dialogue with Trypho is pretty good, as well.
Regards