When does first trimester end ?

Question

So I feel dumb for even asking this, I always assumed that 1st trimester ends at 12 weeks! I found out Friday my OB office considers 1st trimester to end at 14 weeks... what are your opinions ?

Answer

14 weeks is the start of the second trimester.

Answer

I was also told 14 weeks (12 weeks fetal age not counting the follicular phase). Confusing I know. If you do the calculations 40/3=13.3.

Answer

13w6d is the last day of your 1st trimester