Ultrasound/menstral cycle Vs Bloodtest

Hi Mummies, I recently found out that I was pregnant. Based on my 5wks menstral cycle, doc says i shd be abt 4wks plus pregnant. Did an ultrasound scan too and baby is in right position. However, after doing a blood test, it says tt I am likely 2-3wks pregnant. Any idea why the difference? Which is more accurate? Thanks in advance.