How accurate are conception dates given at 7 to 8 weeks gestation??

Very. If done by ultrasound and correlated two to three times in subsequent weeks, it is probably within 2days. If it is a single measurement, it would be less accurate but still within a 4-5 day margin. As always, depends on the skill of the ultrasonographer. In a fertility practice, like ours, that scans early pregnancy frequently, we find this level of accuracy. Hope this helps.