How accurate are early scans?

I had 2 early scans because of expected ectopic. First one they couldn't really see anything so they booked me in a week later. Heartbeat was present at this one and she said I was 5+6 weeks. so I would have been 4+6 at first one. How accurate would this be? I have been monitoring ovulation with opk but this makes it that I conceived a different time from when I ovulated. I'm just a little confused.