If conception date assumes you don't count the first 2 weeks of gestational age, is it still accurate if you don't have an average 28 day cycle?

If from early U/S: If a conception date is estimated from an early ultrasound scan (at 8-11 weeks gestation) the accuracy of the estimate doesn't change even if the cycle lengths are different. This is because the estimate is based on what the fetus looks like (size measurements, etc...), and that depends on how many weeks the fetus has been growing, not on the mom's cycle length.