My scan date says 7 weeks 3 days does that mean conception occurred 5 weeks 3 days ago if your lmp was not known do they measure the sac?

Exactly!: You are correct. The extra 2 weeks are added because of an old convention, and we date the pregnancy by "menstrual weeks". Full term = 40 weeks from lmp. We always use your menstrual age (even if we don't know your lmp) and we never really talk about fetal age (time since conception). I wish we could change this convention, it's confusing! by the way, there is a 4-7 day margin of error w that date.