I am starting mine at 38 weeks, although as I teach, I have a crafty 6 week break now!!
I am only staying on so long as I am a head of department and I have new staff this year so feel it would be a bit mean of me to leave after they have only been there a couple of weeks. Mind you, I was in hospital at 37 weeks with my first and had him by emergancy section at exactly 38 so who knows if I am going to last or not.
It is much easier to trigger an earlier start to maternity than it is to put it back if you are feeling okay.