Arrrggghhh!!! So BB threads are organised such that at the completion of the 13th week, you start second trimester? Or is it the completion of week 13?

I know it is a small and trivial point, but it is just so confusing! I am in my tenth week, am nine weeks pregnant and the magical second trimester mark is at the end of the thirteenth week (when I am thirteen weeks pregnant and a few days, i.e. the fourteenth week). 32 Days to go ????

Is that right?

Would you believe I have tertiary qualifications in mathematics and I still can't work this out.