Can someone please explain?

CCBelATC

I'm not understanding timing. This is my first pregnancy and even though I'm reading books and learning a lot on these boards I still feel clueless. I haven't had my first "real" appt with the nurse yet; only early lab work and an US after I had some bleeding.

I've called the Dr's office twice with a short list of questions to ask the nurse, and one was about the hair highlighting debate. I'm 7w1d now. The nurse said 'if I wanted to be a purist I should wait until the 12th week'. Fine, whatever, can do, but when is that?!

Yesterday for example, my ticker changes and says I'm 7 wks. TB emails me the "your baby at 7 wks" newsletter or whatever it's called. I'm signed up on baby center and a couple other sites, and literally every other site sent me info on the 8th wk...they're always a week ahead. Am I in my 7th wk or the 8th? Also, is the end of the first tri the 12th or the 13th wk?

Can someone explain it to me? TIA!