I'm exactly 40 weeks today and had my first contraction at about 9pm this evening.
I wouldn't have normally thought I was in labour (lots of umming and ahhing over the past two weeks) but since my first pain I've been on the loo 4 times and its what I can only think of as a 'clear out'.
Contraction every 20-25 minutes, lasting around 20 seconds so think I'm in for a loooong night.