Does your third trimester start at 26 or 28 weeks?

McKenzie • first time momma to easton 💙 wife of an airman

I’m 26 weeks today and I’ve had a couple of my apps say I’m starting my third trimester now (this one included) and then the rest of them say it doesn’t start until 28 weeks, so I was wondering when you guys considered yourselves in the third trimester?