Unprotected sex while on birth control.

My fiance and I have been together six years. The whole time we had intercourse I've been on some form of birth control. I went from the birth control shot stayed on that for nine years. After coming home from the military I switched over to the pill. I started having flu like symptoms so I went to the doctor and find out I was pregnant.

I can't remember when I had my last period, so how will the doctors determine how far along I am in my pregnancy?