Third trimester?

Madison • Baby Boy making his way to June 3, 2016 ❥ madly in love with his daddy ❥ blessed.
Okay, this gets so confusing when it comes to this topic but for turning 26 weeks two days ago, the Nuture app says I'm now in my third trimester, but I've been told different and the third trimester starts when you turn 27 weeks, also been told that's when you turn 7 months pregnant . Which one is true?