Lol I honestly don't really know. Like I said all I know is right when I hit 13 weeks it said second trimester. I just keep track of weeks, I don't pay attention to months and trimesters.
The app says 1st trimester until you're 13w +2 then it says 2nd trimester
I thought it was end of the first? Lol
When I hit 13 weeks this app told me it was the second trimester