The Myth is/isn't a Myth: 1st Trimester

So...which one is it? In several different books I have {What To Expect, The Pregnancy Countdown, my Pregnancy Journal, the Doctor's Handbook, thebump.com} there seems to be a different opinion on when the first trimester ends: is it 12 weeks or 15 weeks?

If a woman is pregnant for 40 weeks, and you split that up into sections of 3...wouldn't it be 12 weeks?

Who knows! All I know is...there wasn't a 'light-switch' affect for me, but there has definitely been a 'slowing down' of sickness, so I will take that! I'm at 15 weeks {so working on my 16th week} so I'm hoping to begin cooking a lot more this week and hopefully even making some meat dishes now! haha!

I think I'm officially into the Honeymoon Trimester as the books says!

What do you all think?!