I don't remember being so confused about trimesters in my first 3 pregnancies, but I am now. I haven't talked to the doc to get her input yet and probably won't until my next appt in February, so just wanted to see what the general consensus is here.
Babyzone & Pregnology break them out like this: 1-12 / 13-27 / 28+ (Using this one us Aug. 5th EDDers begin our 2nd trimester on Sunday.)
What to Expect breaks them out like this: 1-13 / 14-27 / 28+
But then I found this site, that has 3 different trimester timeframes: http://www.baby2see.com/trimester_calculator.html 
2nd trimester either begins at 12w (gestation) or 13w3d (development) or 14w5d (conception).
3rd trimester either begins at 27w (gestation) or 26w5d (development) or 27w3d (conception).
So. I'm just curious what you think or what your doctor has said. I could call the nurse and ask, but I hate taking up her time with a silly question when someone else who may really need her is trying to get through.