I have heard 25-29 weeks so I myself was going to  start at 27 weeks since that in the middle. I went by 1-13 weeks for the first trimester and am going by 14-26 weeks for the second and 27-40 weeks as the third trimester. 

Thats how I did it too since different books and websites give u different answers about that stuff. and even drs give different answers too. i loved starting my 3rd trimester lol made me feel like i was almost done.. Now im less then 2 weeks away from having her and im excited.