How accurate are ultrasounds in determining due date?

Depends on dates: The ultrasound (us) is the most accurate way we have of deciding how pregnant you are. There is a variation in us accuracy due to how we measure and who is measuring, but the accuracy is about 3-4 days before or after the measured size until about 10 weeks, plus or minus a week to about 14-15 weeks, then gradually changing up to a 3 week range in the last 2 months of pregnancy.