How many months am I?

Kay • 3rd baby on the way and it's a Girl!
I am so confused about how many month I am. I say I am eight months because I am 31 weeks pregnant but according to my app it says I am 7 months. I need some help figuring this thing out. Also when I tell people I am due on February 9th. 2016 they look at me and say but your 8 months how are due in February? Can some one give me a one liner to use? Maybe I'll make another post about that question.