The virgin birth of Jesus appears to be a borrowing of the virgin birth meme found in many pre-christian myths about the births of demigods. Note too that Matthew 1:23 appears to deliberately twist Isaiah 7:14 to support the ingrafting of this pagan virgin birth meme into christianity. Isaiah uses the word "maiden" meaning simply a young woman. But Matthew twists it to say virgin. So the whole virgin birth story of Jesus is based on what appears to be a deliberately false transation/quoting of Isaiah 7:14 in an effort to incorporate a popular pagan meme into the new christian cult. In many respects christianity is a sychretic religion that combines Judaism with pre-existing pagan traditions. A lot of the pagan traditions are in the form of pagan stories rewritten within the gospel narratives with tweaks here and there as they're applied to Jesus. But most christians are unaware of this because they are unaware of these ancient pagan traditions and stories that clearly inspired the NT.
This video is recommended viewing (fast forward to the 27:45 mark where actual lecture starts):
http://www.youtube.com/watch?v=ILldt2XHZw0