Trimester confusion

Leah • 32, Accountant.
Nuture is saying that at 26 weeks I am in the third trimester. Yet other sources say the third trimester begins at 28 weeks! 
Plus it states on another app that I am now in my 7th month of pregnancy. But with baby being due in May I have 3 months left... so are we pregnant for 10 months realistically? 
I'm so confused!