The ending of the first trimester

Lindsey • A 6yo, 3yo and … Hubby had a failed vasectomy 🤣
So I know the general thought is "I made it to 12! First trimester is over!" But from other apps I've seen that week 13 is the last week of the first and week 14 is the first week of the 2nd?  I'm 11 weeks today and now I'm confused. I thought I was so close and 2 more weeks seems forever away!