When does 1st trimester end and 2nd begin?

Mitzi Bishi

I know there are a lot of different ways to calculate, but what did/do you consider the end of the first trimester? 

By development, gestation, or conception?  

I'm 12 weeks today with an appointment on Tuesday. We don't really know when to announce to friends or extended family. Part of me wants to keep this secret bottled up a bit longer, but it's slowly feeling a little more real.