How do I know how far I am?

I had a miscarriage back in November the 23rd and I tested positive agian for pregnancy the 17th of December (doc confirmed new pregnancy) I still have to wait 2 more weeks for a u/s after my first appt

But my question is how would I know how far along I am as of now? Would I just count when I started bleeding as cycle day 1 like if it was a period?