Which is the date of conception on an ultrasound?

No. Gestational age is measured from the woman's last period. Even if she doesn't know when this was, the gestational age is time from conception plus 2 weeks. For example, a 'gaedd 19 weeks' will give you a due date 21 weeks in the future. You are 17(+ 1 day) weeks from conception. 19(+ 1 day) weeks from your notional last period and 21 (- 1 day) weeks until your EDD.