A typical pregnancy is 280 days from LMP which is 40 weeks. 40 weeks divided by 3 trimesters is 13.33 so between weeks 13 and 14 you're hit the second trimester.
Ps @baby3due2016 ... If you're hoping morning sickness will end as soon as you hit second tri, you could be disappointed! Just being realistic... I was sick as a dog til week 17
Re: When does it start?