When it it REALLY the end of the 1st Tri?

allclad13

This is my first pregnancy so I assumed that the end of the first trimester is 12 weeks.  But now I keep reading/hearing that it's not until week 14.  So what's the deal? 

I don't really care when it's officially the start of the 2nd trimester but I would really like to know when the miscarriage rate goes down. Do they consider that at week 12 or week 14?