The Trimester Breakdown

I'm getting really confused about how far a long I actually am. There are so many different breakdowns of week by week and what weeks start a new trimester. Some say the second trimester ends with the 26th week. Other calculations say it ends with the 28th week. So which is it? Over night I went from thinking i was in my 5th month to being in my 6th month!

I guess which month I'm in is less important than the week. After all, it is still a 40 week countdown despite which month the weeks fall in. Today, I'm 23 weeks and 7 days!