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40 weeks divided by 3 is 13.333.... Considering the average mom delivers at 41 weeks and 1 day, I say the start of the 14th week. But I guess if it makes you feel better, you could say the start of week 13. Anyway, I really don't know where 12 comes into play...
my only guess as to why 12 comes into play is because it's referring to the 2nd trimester beginning when you FINISH the 12th week.
i have searched the internet high and low and all the reputable sites i can find say either the END of the 12th week (so, 13 weeks exactly) or the END of the 13th week (so, 14 weeks exactly). i am not sure why there is conflicting info out there! i think it all has to do with age by gestation (counting the 2 weeks before you conceived) versus age by conception (counting beginning the day of conception).
i, personally, consider my 2nd trimester to begin when i am 13 weeks exactly.