FIRST TIME MOM! Lol

Tyler • Mommy of an angel baby👼🏼 baby number two is due March 8, 2017 so blessed and thankful! Thank you Jesus!
Sorry I'm a first time mom so I'm a little confused on when I start my second trimester. I'm 12 weeks 1 day today. 
Did I start my second trimester when I hit twelve weeks. 
Or do I hit my second trimester after this week. When I'm 13 weeks ! 
NO JUDGING PLEASE!