You will get many different answers from this site, just go with what your doctor or mw uses.
Its 13 & this I'm expecting app says it as well...2nd is week 13-28 I learned in school & many books & websites such as http://****t.com/second.htm
It starts after you have completed your 13th week, so when you are 14weeks 0 days
its starts from 14weeks :)
the "what to expect when your expecting" says 13 and so does similac and a few others
This app says week 13 starts your second trimester!