When does 2nd trimester *really* start??

Marion

I’m finally in my 12th week and counting the days until I’m in my 2nd. That said, I’ve read online different things for when it actually starts. Some say week 13, and some say it doesn’t really start until 14 weeks.

But then the <a href="https://play.google.com/store/apps/details?id=com.glow.android">Glow App</a> post for today (12 weeks, 1 day) says, “Now that you’re in your 2nd trimester you may want to start telling people.” What?!? Anyone know???