I've seen different websites list it as early as 12 weeks, and as late as 14 weeks.
I did a 'trimester calculator
' online a couple of days ago, and it told me that I would start the 2nd trimester on January 24, 2012. That's today, so I'm going with that. That calculator divides the 40 weeks of gestation into 3 equal parts of 13 1/3 weeks each. I like that. Nice, neat, easy, equal parts.
What do you ladies think? When do you think that the 2nd trimester starts?