When does the 1st trimester end?

Kaitlin
So I've been reading different things. One I read that says 12 weeks is the start of the 2nd trimester. One I read that the start of 13 weeks starts the 2nd trimester. And I also read that 14 weeks or the end of 14 weeks is the 2nd trimester! Anyone know for sure? Thanks in advance!!!

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