How accurate are early sonograms to date a pregnancy?

Ultrasound dating: All due dates are estimates and OB providers typically use the due date calculated from the last normal period to determine a woman's due date. An early ultrasound done between 6 and 9 weeks of pregnancy is considered to be accurate to within 5 days of a due date. If this ultrasound differs by more than 5 days from the period due date then the ultrasound due date is used.