Trimesters

I feel dumb asking this but I've seen and heard so many different answers! When are you considered to be in your second trimester? Isn't it month 1-3 is the first trimester, months 4-6 the second trimester and months 7-9 the third trimester? So wouldn't that mean that week 12 means you're in your second trimester? I read somewhere that week 14 is considered the start of your second trimester.