26 weeks and Glow says I'm in my 3rd trimester?

Pamela • Son Patrick 3/6/06. Chemical pregnancy 11/2016. Baby girl/boy twins Liliana and Alexander 11/7/17

I always thought 27 weeks started your third trimester. Some even say 28 weeks. Is this because I am having twins and won't be going to 40 weeks? One of my doctor's partners saw me at 11 weeks and said he considered me in my second trimester. Should I go by what Glow says? If I make it to my schedule delivery date I only have 12 weeks to go.