How far along am I?

Hey, I recently found out I am pregnant. I'm travelling Europe and won't be back home for another month and apparently because I don't have a GP here they can't really do anything. Anyways I'm trying to figure out how far along I am. My last period I had was December 29th to whenever it ended. My next period was meant to be January 30th but I missed it as a symptom of my pregnancy... I had sex on January 20th which is how I got pregnant. So my question is, do I count my weeks from the last period I actually had even though I was never pregnant until January 20th or after that date or the period I missed?