What do you count as third trimester?

Question

I've seen a lot for 27 weeks but 28 makes more sense to me, 12 weeks to go / solid 3 months. 
And Glow says 26 weeks but I don't see anywhere else that says the same. 
Not super important but I just like to see what everyone thinks :)

Answer

I go off that.

Answer

It goes by completed weeks.  At 27 weeks and one day you are in your 28th week and that is when my doctor said I was in my third trimester...if that makes sense

Answer

I thought it was 28 weeks. But I've seen 27 too.

Answer

It's every 13 weeks a new trimester starts. So beginning of week 14 you've officially started second trimester, at the beginning of week 27 your in your third trimester.
1-13 first, 14-26, 27 till 40.

What do you count as third trimester?

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