Ah, you refer to the prophecy in Isaiah 7:14.
Isaiah is in the OT, and written in Hebrew. I am sure they know their language better than we do. The word almah, which is what was used in the Hebrew, does not mean virgin. It means young maiden. The Hebrew word for a virgin is betulah, and it was not used in that verse. Jewish scholars have been saying that for centuries.
In 2011, both the U.S. Council of Catholic Bishops and the NIV's Committee on Biblical Translation both agreed, after extensive research, that the Jewish interpretation into English is more accurate. They are revising the verse to read "a young maiden" and omitting the word virgin. You can thank Jerome and his 4th century Latin Vulgate for the bad translation.
That does not mean that Mary was not a virgin. You don't need the prophecy in Isaiah. The Book of Matthew implies it when the angel visits Joseph and tells him not to put her away in shame, after Joseph finds out that she is pregnant. The angel tells him that she carries the son of God, and from there everything is hunky dory. That is what I refer to. They can change Isaiah all they want to.
The Septuagint translated this word virgin in the greek language. The Jews reject Jesus as the messiah. He is their curse word. A huge part of Jesus ministry was trying to help the people understand the truth in scripture rather than the screwed up interpeation of the Jewish Religious Scholars who "put" themselves in the seat of Moses.
Jesus quoted the LXX often and never once did he or any of his disciples dispute "the translation" of the word to a clear word "virgin". Not only that but the new testament itself confirms that defination.
We also know the disciples and the early Chruch fathers ALL confirm that understanding.
It's one of the foundational teachings in the Christian faith.
New Testament cleans up any doubts a Believer may have.
'Behold, the virgin shall be with child, and shall bear a Son, and they shall call His name Immanuel,' which translated means, 'God with us,'†Matthew 1:23
"For in Him all the fullness of Deity dwells in bodily form," Colossians 2:9