Last period doesn't really work either though outside of trying to date people before their dating scan. My last period was April 30 but I'm only due the day before you because I have longer cycles, so by using that method of calculation I would have a longer pregnancy than other people going off due date, but that's obviously not the case –my pregnancy 'length' is the same as everyone else's!
Being XX months pregnant is only something you use when describing to people roughly how far along you are as you don't generally use weeks outside of clinical settings (I wouldn't say to my gran that I'm 26 weeks, I'd say I'm about six months) - it's not actually an accurate measurement and can't really translate to trimesters in that way.