Its one of those things that there will never be a quantative answer for. I know you have a scientific background, but not everything can be answered scientifically kwim? There are several thoughts about this, somewhat like the 'chicken or the egg' question. Some say that the baby instigates the labour and others say the mother, and some say both together. It's true that even if a baby is ready that the mother can delay the process by not letting go emotionally - many women who've gone past dates go into labour on their own once an induction or c/s date has been booked - almost signalling that it's *safe* to give birth. I personally believe that our mind plays a huge role and we have to stop thinking with our 'concious' brain when it comes to birth and think with our 'primative' one (there is something on here about it that Kelly posted some time ago).

The giving of an EDD is just that - an ESTIMATED date that baby should arrive, but it is accepted that term is anything from 37 to 42 weeks (some women even go to 44) so as long as your baby is born within that time frame then there is no cause for concern really, baby will come when it's time to come. And even then the giving of an EDD would only be a recent thing that I would assume would coincide with when birth became medicalised and hospitals and Drs had to be booked . Decades ago a woman didn't even acknowledge the pg till she had felt the 'quickening' (first movements) and only worked on a rough guide for when the babe should be born. Many factors can alter your EDD too - length of cycles, day of ovulation and then implantation. Some babies need to gestate for longer to be fully developed and others need a shorter time. There's really no science to it, its just one of those things that is completely beyond prediction.