HCG and last period how far along em I ? help please

Lauren
Please help, first time pregnancy .So my last period was over 5 weeks ago and the HCG blood test estimates that I'm around 3-4 weeks along but my dr said when I first saw him that I was 5 weeks as you go from the first day of your last period so when I think how far along I em , em I 3-4 or 5 weeks along and when should I get my first scan please help 😀