With DD1 I had 6 hours of back pain (she was posterior) and then my waters broke. She was born 8 hours later. I didn't associate the back pain with labour though because I was only 36 weeks and wasn't expecting to go into labour.

DD2, I had one strong pain all of a sudden and then gave birth to her four hours later. No warning at all.

One thing I have found is that a lot of women have days or even weeks of pre labour and don't even realise it's pre labour. I have also found from personal experience, that when you stop thinking "when is it going to happen!?!" it happens....murphy's law good luck with it all xx