When did I conceive?!

On the 8th of November I went for my first scan and I found out I was 7 weeks pregnant but I don't know my due date. I've heard something about the midwife adds 2 weeks on but I don't get it and I really need to know when I roughly conceived. So from the 8th would it have been 7 weeks ago, or add on 2 weeks or minus 2 weeks?? I hope someone can make sense of this, it's important😂

I had sex with 2 different guys 2 weeks apart, so finding this out will help me determine the father.