Agreed! Every site has something different varying a week or so :( I just go by this app
On this app it says 13 weeks for 2nd trim. And 27 weeks for 3rd trim.
In my pregnancy book it says 14 weeks for 2nd trim. And 28 weeks for 3rd trim.
I'm not exactly sure why some say different weeks but whatever one you want to go by is fine. :)
Just go by six months from your due date, and then three months from your due date. I'm due Jan 29, so I started my second trimester July 29, six months before my due date. I was two days short of 14 weeks. I'll be in my second trimester October 29. I'll be one day short of 27 weeks.