How to date your pregnancy? Dr has confused me!

Hi all, so according to my apps, I am 5 weeks 3 days pregnant. My last period started on 3rd December, and so I've dated my pregnancy from that date. One doctor confirmed that is the correct way to date it, but one said I should be dating it from my day of conception? When I had my early scan due to some complications, I thought I was 5 weeks, but the scenographer said as she could only see something very small I was less than that! So confused... Can anyone please confirm how I should be dating my pregnancy? Thank you!