might be a stupid question!

So we are currently pregnant with our rainbow baby after we had a missed miscarriage in Dec 16 with triplets. When the doctors work out how far you are gone they do it from first day of last period which would make me 7w 6days but from day of conception I am 5w 6 days - I just wanted to know when you go for an ultrasound would the baby be 5w 6 days or do they use measurements to show that I'm 7w 6 day? Hope this makes sense after what happened last time I don't want to have a scan too early as I don't want to go through what we did last time