I'm due Nov 4. And I have a few apps on my phone and one says 25 weeks starts the 3rd but all my others say either 27 or 28 weeks is the start. I don't really feel like I'm in the 3rd until I start going to the dr weekly at 30weeks lol
I have always read and believed this:
First trimester - 1-13 weeks
Second trimester - 14 to 27 weeks
Third trimester - 28 to 40 weeks.
So, once you complete 27, you would start your third trimester.
According to the literature I got from the doctor's 25 weeks is the third trimester