Anyone who has read Matthew and Luke notices that although they often have different emphasis, they are pretty much telling the same story. The story of Jesus is recongnizable. Some of the differences are because of a particular emphasis. Luke, for example, is very much focused on concrete poverty and many of his stories include an exhortation to take care of the poor. So where Matthew has "blessed are the poor in spirit" Luke has "blessed are the poor." Each tries to capture and convey in the limited space at their disposal a fuller picture of Jesus. If I can read an entire Gospel in a couple of hours it is safe to think that Jesus' 30+ years of life are not totally contained in the twenty or so chapters!
This brings us to Matthew and Luke and their infancy narratives. First of all, Mark does not have any stories about baby Jesus. He starts off with John the Baptist. There are lots of theories why this was so. No one knows. There is good reason to think Matthew and Luke actually had a copy of Mark's Gospel in their possession when they wrote their own Gospels. The arguments make sense, but I also know life is less logical than our reconstructions are. At any rate, Matthew and Luke have two stories of the birth of Jesus which really do not easily fit together. One reason for this, perhaps the main reason, is probably because they are telling the birth story to make clear what Jesus' life really means from their perspective. We forget that ancient history had different expectations and different emphases from our history. For one thing there was an understanding that the narrative was also a construction where the author had some leeway to construct his story. [Truth be told even today that is still true. Anyone who reads a newspaper account knows that the writer leaves out many details and plays up an angle for the story. History is always written from a perspective and the writing always conveys a view point.] However, there is still more or less accurate writing and even ancient authors were expected to write in a way that respected the truth.
As I said, the birth stories are different but both agree that Jesus' mother, Mary, was a virgin. The question which I have is this, if Mark does not mention it then where did Matthew and Luke come across this idea? Compounding this is the rumor of Jesus' father being a Roman soldier. This means that in ancient times the birth of Jesus was thought to be under unusual circumstances and this was used by enemies of the Christian church. So something to think about as we enter the final week before Christmas. What does it mean that this man's birth was to a virign?