NPNF2-06. Jerome: The Principal Works of St. Jerome - Christian Classics Ethereal Library32. Isaiah tells of the mystery of our faith and hope:4478Behold a virgin shall conceive, and bear a son, and shall call his name Emmanuel. I know that the Jews are accustomed to meet us with the objection that in Hebrew the word Almah does not mean a virgin, but a young woman. And, to speak truth, a virgin is properly called Bethulah, but a young woman, or a girl, is not Almah, but Naarah!4479What then is the meaning of Almah? A hidden virgin, that is, not merely virgin, but a virgin and something more, because not every virgin is hidden, shut off from the occasional sight of men.
The above is St. Jerome. He knows that his translation is wrong but carries on. Otherwise, they can't use Isiah 7:14.
A young woman that conceived is a million times a day event but not a virgin conceiving.