Trimister one or trimister 2?

Shivi
I am just a little confused... Is completing 12 weeks is considered trimister 2 or completing week 13 is trimister 2. I am 11 weeks now and wanted to know this . Different sources says it different. Also if I am on my 11 th week plus few days ... Is week 11 or week 12?  I am pretty sure it's week 11 but just wanted to know what others have thier calculation!!