The Virgin Birth

From where did the Christian idea of the virgin birth come? Matthew 1:23 says, "The virgin will be with child and will give birth to a son, and they will call him Immanuel..." This is a quote of Isaiah 7:14. The Hebrew word used in Isaiah is almah which is understood as young woman in most places in the Old Testament. In fact, there is a word in Hebrew for virgin. The word is betulah. Why didn't the writer of Isaiah use betulah to avoid any Jewish-Christian conflict or misunderstanding of this verse? My Jewish professor, this week, tried to explain how the Christians misinterpret Isaiah 7:14 for their own purpose to prove the Messiahship of Jesus. The fact, according to my professor, is that almah was mistranslated in the Septuagint (the Greek translation of the Old Testatment) as virgin with the Greek word parthenos. This argument could hold water for my professor except for the fact that the Septuagint translation was written between 240 B.C. and the first century B.C. by 70 Jewish sages. That is, before Christ or Christians, for that matter, were even on the scene. There was no Jewish-Christian conflict yet. The Septuagint translated almah as virgin in other places, as well. To imply that almah only means a young woman is to be a bit dishonest. In fact, in Genesis 24:16, 43 the Hebrew words almah and betulah are each used to refer to one person, Rebekah, the young virgin who was to be wed to Isaac. The Greek translated both as virgin, which is not a misrepresentation of the usage by any means. Is it possible that almah, as it was understood then, referred to a young woman who was eligible to be married was also understood to be a virgin?