When does Trimester 2 start?

Stephanie

I was always under the impression that T2 starts at 13 weeks. Why does everyone say to wait until you are 12 weeks and out of the first trimester to tell people then? Also, my favorite pregnancy book, Pregnancy Week but Week, says T2 starts IN the 13th week, so 12 weeks and 1 day. What the heck? When does T2 start?! I am 10 weeks 2 days and can't wait to be out of the first trimester!