So frustrated with this months vs. Weeks crap.

I dont understand the whole 1st trimester is 14 weeks 2nd is 14 weeks as well and the 3rd is only 12 weeks. Where did this come from? This was not the case 7 years ago when I had my daughter. It was simple. Everyone knew how many months they were. Now some people would say I'm 5, 6 or even 7 months.