So when does 2nd tri actually start??

I thought that this was a really simple question but I've been given 3 different answers!! 

I previously thought it was the end of the 13 week - so 14-0.  But I subscribe to http://www.babycentre.co.uk/ and each week they send me an email saying what stage I am at based on the first day of my last menstrual cycle.  The email I received today says that I am already in 2nd tri and I am 12 weeks 3 days today (or in my 13th week). 

I did some googling and the answer seems to be different on each of the websites I look at!  Ultimately it doesn't make much difference except in perception - the rate of miscarriage drops massively once you are in 2nd tri so it's a big milestone for most women and one that I have been anticipating.  I think it deserves some celebratory chocolate at least! 

What do you think?  Does 2nd tri start at 12, 13 or 14 weeks?

Thanks very much!!