What is the conception date if the ultrasound today says the fetus is 8 weeks and 4 days old?

Count back 2 weeks, so 6 weeks and 4 days old. Count back 6 weeks and 4 days old on a calendar and that would be an estimate. Also, check out www.babymed.com, and put in you're due date or you're last LMP and they can tell you an estimate of conception. God Bless