I had my second trimester celebration challenged by a friend and had to look up the official cut off points:
The Developmental second trimester starts at 12 weeks (third at 27 weeks), when it officially becomes a fetus and the scary miscarriage rates reduce.
The Gestational second trimester starts at 13 weeks + 3 days (third at 26 weeks + 6 days), which is the 40 weeks divided equally into three trimesters.
The Conceptional second trimester starts at 14 weeks + 5 days (third at 27 weeks + 3 days), which is the true pregnancy weeks (38) divided into three trimesters.
As I am 12 weeks today, I say 
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