20 Weeks Pregnant - halfway there? A discussion on due dates

A common comment to hear at 20 weeks pregnant is, “Oh, so you're halfway there!” This is probably roughly true, but let’s talk about due dates! Firstly, have you ever wondered who decided that a pregnancy lasts 40 weeks, and how they came to that conclusion? And most of all, how on earth is it that 40 weeks has come to be seen as a deadline, as if the mother or baby are all of a sudden at high risk from one day to the next?