trimesters ....

marie_144 • Jace michael born December 12, 2017 ♥️ Baby #2 due January 21,2020 💕😁
So I'm a little confused , I'm 10 weeks and 3 days today , as of Monday I will be 11 weeks ❤️😁 but I'm a little confused as to when 2nd trimester starts ..... I've heard 11 weeks , 12 weeks , and 13 weeks .... so when does it actually start ?