Weeks Calculation!

My last period ended on 1st Dec, I had sex with my hubby on the 8th Dec. It gave me positive reading on a pregnency test at home which I did on the 29th Dec it said 3 week plus. Shouldnt I be on 4 weeks 6 days instead on <a href="https://play.google.com/store/apps/details?id=com.glow.android.nurture">nurture</a> app it claims I am 5 weeks 5 days.

I am confused!