"Critical problems with regard to the meaning of the term "virgin" involving philological exegesis of the text have now been definitively resolved. Rolla writes: 'The Virgin (in Hebrew ha'almah): the presence of the article indicates that the person in question is someone definite, if not from the context, at least in the prophet's mind. The person in question is 'almah, that is, a young girl, a girl of marriageable age, maiden, virgin. In fact, the Hebrew term, in the eight places where it does appear in the Hebrew Bible apart from the present text (Esther 2:8; Gen 24:43; 1 Chron 15:20; Song 1:3, 6:8; Ps 45:1, 67:26; Prov 30:19), always indicates a woman both sexually and chronologically mature, not named, and, therefore, ordinarily still a virgin."
-Fr. Stefano Manelli in All Generations Shall Call Me Blessed: Biblical Mariology.
Not to mention the exegesis St. Matthew does in his Gospel (which is the inspired word of God):
"All this was done in order to fulfill that which the Lord had spoken by the prophet: Behold, a virgin shall be with child and bring forth a son who will be called Emmanuel, which means God with us." (Mt 1:22-23)
St. Ambrose also points out that Isaiah 7:14 is evidence for Mary's virginity remaining intact during the very act of giving birth:
"This is the Virgin who conceived in her womb, the Virgin who bore a son. For thus it is written: Behold, a virgin shall conceive and bear a son. Not only is it said that the virgin would conceive, but that the virgin would bear a child."
The Fathers of the Church also compared the miracle of Mary's virginity remaining intact in giving birth to Christ with the example of light shining through a glass.