Getting exceedingly p'd off with the whole EDD thing. 38 + 6 today and baby theoretically due next Saturday. But of course could have been born at any point in preceding two weeks, or two weeks after.
Which is a bit crap really. They probably predicted baby's due date with as much accuracy 500 years ago.
Surely there must be some statistical analysis taking into account baby's size, previous pregnancies, mothers weight/height, genetic indicators etc which would let you predict with greater accuracy than 'some point within x four weeks'.
Bet if it was men that got pregnant, your due date would be calculable down to the hour and you'd receive daily count down text reminders! 
As you can tell, I'm a bit fed up!