Therefore the Lord himself shall give you a sign
The passage doesn't prophesy a miracle! It maintains that the threat from Israel and Syria will be wiped out before the kid knows his right hand from his left--- read the context of the passage- there is NOTHING prophetic of Christ there!
Furthermore, even the word alma does not imply virginity. Modern scholars say the word simply means young woman, not virgin.
https://en.wikipedia.org/wiki/Almah
See also the conclusion:
"The first issue is the translation of the Hebrew word almah. Most scholars agree that its general meaning is "young woman" and it rather looks like the choice of "virgin" by the KJV was theologically motivated. [...] Indeed, we do not know of any Jewish writing which understood Isa 7:14 as involving anything but a normal conception."