If you take 40 weeks divided
by 3 you get 13.3 weeks. So, I think, by the time you end your 13th week, you should be consider in your 2nd trimester. :) Most people still use 12 weeks, but that only takes you to 36 weeks and a pregnancy, usually as you know, is 40 weeks. :) Not sure if this helped or not, but it's the only way it makes sense to me.