Dating Scan?

The last day of AF was the 8th of June. The doctor I saw today says that means I'm 3 weeks and 1 day pregnant today. So I got a referral for a dating scan while I was there and she told me to wait until I'm 6 weeks to get the scan. So is that 6 weeks since my period or from conception? I couldn't understand her very well so I didn't think to ask her this. So can someone help? Should I book it for 3 weeks from today? Or should I make it 4 weeks to be safe?