When does the Second Trimester Start???

Ok I know silly question but I have read 3 different things and now I have no idea! Fertility Friend is telling me I'm in my Second Trimester now, being just over 12 weeks. Then I have read that it starts at the end of 12 weeks, so I guess that is 13 weeks! Then I have a book (it was published in the USA if that makes any differnce) which says that the second Trimester is not until week 14........... Can someone clear this up for me??
Thanks in advance!!!

i take it as at 13 weeks (which is the start of week 14)

thanks for that

I always thought it was the start of wk14 and 3rd tri is at 27wks. I hear you on the conflicting info though. Everything says something different!!

Beginning of week 13. So 12 completed weeks is the end of first trimester, and day one of week 13 is the beginning of second tri.