When does the 2nd trimester start?

Question

I've seen one place say 12 or 13 and others say 14 and I'm confused as to when the 2nd trimester begins now

Answer

There are four weeks in the month trimester typically means the end of every third month" so the 13th week is the start of the second trimester.  Source

Answer

I was reading this book last night and had this same question it seems like anywhere between 13 and 14 would start the second trimester I guess based on when you conceived like right now I'm scheduling to go into 12 weeks but I conceived in December so it almost seems like I'm in my second trimester already