I am not sure about the science of this. But i was born 3 weeks past my EDD and my mum had to be induced so didn't go into spontanious labour even after 43 weeks. I have heard of a women going up to 44 weeks and then went into spontanious labour. Some babies may just need a little longer to cook but modern doctors only let us get to around 10 days past. So who knows if they could have gone into labour even the next day, next week, etc.