When are prenatal ultrasounds most accurate for estimating conception? 6 weeks or 10 weeks or 19 weeks?

I vote for: 10 weeks: Usually, prenatal ultrasound scans done while 8-11 weeks pregnant give accurate dating estimates. All dates are estimates, but that doesn't matter much in most cases (an estimate is close enough). If one's cycles are long (more than 31 days) or short (less than 25 days), be sure that is taken into account when making the estimates.