It actually starts at 28 weeks
Going off this app alone it's week 27
Actually, going off the app, its in the middle of 26 weeks. It really depends on how you estimate it. Growth, OBGYN, and midwives all do it differently. It's so weird. I like to go based on the app because it keeps up with what the videos tell me in development. Haha
Going by this app and babycentre and also my healthy pregnancy guide it's 27weeks.
I asked this awhile ago and got mixed answers. I also Googled it came up with various answers. Even the different books I have all say different things. This app said I was third trimester at 26+4. Some books say 27 and some say 28. I'm now a little over 27 weeks but just saying I'm in my 3rd trimester. It's easier lol