Thanks for the replies so far. I'm still hoping that someone will have some statistics on this.

thepixie, that's a good point that there are more weeks available to be born early than late. Hmm, if the due date is the "average" day a baby will be born on, it depends what kind of average.

If it's the median date, then by definition, 50% of babies would be born before then and 50% after then.

But if it's the mean date, then the early babies, since they can be earlier than the late babies, would pull the average earlier, so the due date would be actually a bit early, which seems to be common.

I was also wondering later on, what is it that makes a baby be born? There must be some kind of hormonal trigger in the baby or both that says "I'm ready to come out, let's do this birth thing", or in the Mum that says "I've had enough of carrying you around inside". Do we know what that is and how it works?