Now we know that in normal circumstances an unfertilised egg is barren - it doesn't divide and grow, it just decomposes. We also know that even if it did begin to grow of its own accord, unfertilised, the outcome would be a girl, since it is the man who provides the Y chromosome. So, in the absence of male sperm, how did her egg get fertilised, and the required male DNA enter the ovum?
This problem leads sceptics, particularly those of a scientific persuasion, to dismiss this story as a "mere myth", a logical impossibility. We know of no way that a fully human person (or indeed any sort of person) can grow from an unfertilised egg. Within the bounds of science they are clearly right. How, then, is it that highly intelligent people, who know how babies get made, continue to believe this story? I think there are a number of possible answers - take your pick, some or all may be true.
- You could just say "I don't understand it but I believe it happened because I believe in the Bible (or the teachings of the church) and God will make it all clear in heaven". I think this is fair enough - no doubt a god who created the whole universe could solve this little problem too - but I find it a little too glib.
- You could say that God fertilised the ovum himself, so that Jesus carried God's DNA. This could be what is suggested by the angel's words in Luke, "the Holy Spirit will come upon you, and the power of the Most High will overshadow you". This raises some very interesting questions - does God have DNA? I could speculate about this for pages but I won't.
- You could apply the traditional Catholic reasoning about the communion to the virgin birth. The Catholic church teaches that at communion the bread and wine actually become (as opposed to merely representing) Jesus' body and blood. They do not change their appearance - they still look, feel, smell and taste like bread and wine - but their essence changes. Perhaps something like this happened with Jesus - he still looked, smelt and sounded like a human but in essence he was God. Essentially, it's a mystical technique for believing two mutually exclusive things which pretty much describes the dual nature (God/man) of Jesus - so it could just be right.
- You could work backwards - Jesus showed himself to be God's Son in many ways, not least through his death and resurrection. How then could he just be born in an ordinary way? The story of the virgin birth is then a way to illustrate Jesus special relationship with God - the writers may not have intended us to take it literally (other great figures in history, like Alexander the Great, have also been given virgin births by their biographers for this reason) but they intended it as a way of saying right at the start of the story that this is someone out of the ordinary.