Here is a link to a great blog post on due dates. It is called "Poop on Due Dates"
I see such a stark contrast between births where women and their providers are more relaxed about when babies come and ones that expect the baby to come on or before this arbitrary date. I see the stress it causes women and have actually felt this before myself eventhough I have never gone post dates. Read this article, it is full of information and is very accurate.