Pregnancy ultrasound dating accuracy?

Depends: Upon when it is done. The earlier it is the more accurate the dating.

Earlier the better: In the first trimester (< 14 wks) it's accurate within days. The later in pregnancy, the more inaccurate it is to date a pregnancy. Roughly, if an ultrasound differs by a week in the first trimester, dates are changed. 2 weeks for second trimester & 3 wks difference in the third trimester to change the due date compared to an LMP due date. Always keep the earliest us edc, it should never change.