See here: http://en.wikipedia.org/wiki/Virgin_birth_of_Jesus
>The word "virgin" in Christian translations is disputed. The Hebrew word is "עלמה" (almah), which scholars agree means a young woman of child-bearing age, "but has nothing to do with whether she is a virgin", and the context of the passage makes it clear that Isaiah has in mind events in his and Ahaz's near future. The Greek-speaking author of Matthew, however, used the Greek translation of Isaiah, in which the word is given as "παρθένος", parthenos, meaning a virgin.[57] The Encyclopedia Judaica calls this "a two-millennium misunderstanding of Isaiah 7:14", which "indicates nothing concerning the chastity of the woman in question".[50]