due dates change due to measurements???

Tara
Hi to anybody that could help me...
My original dates are 8 weeks and 1 day today. I had a scan today and my measurements show that in fact I am 6 weeks + 5 Days.
Which is more accurate because my last period was 11th January and it is now the 9th of March which is Basically 2 months ago...
Am I more 6 weeks+5Days or 
8 weeks+1 Day?
Thankyou