Months and trimester questions?

Cierra

Each trimester contains 3 months starting from the first day of last cycle

Ik I'm in the second trimester bc I'm 20+1 weeks pregnant but how many months is that? I find it easier to to go by weeks instead of trying to figure out how many months that is. When does the third trimester start? How many weeks/months do I have til the third trimester starts?