With this pregnancy I've had scans at 4, 7 and 8 weeks.
With my DD I had scans at 6, 8, 10, 12, 16, 20, 24, 32, 34, 36, 38
With my other two pregnancies I had 5, 6 and then 6, 8.
So when it comes to scans while I'm not a sonographer I'm a bit of an pro from the lying on the bed point of view!
With early scans I saw nothing at 4 weeks, gestational sac + yolk + foetal pole at five weeks and then have always seen a heartbeat from 6 weeks onwards (even when I've then miscarried). I know a few people who haven't, but did see one a week later.
I was quite surprised by the difference with this pregnancy between 7 and 8 weeks - it was still a blob rather than distinctive baby though. By 10 weeks my DD looked more baby shaped though.
I have a fair few complications and I'm also diabetic with the nicest consultant team you could hope for. I see them fortnightly and they are quite happy for me to be scanned as part of the appointment, hence the number I've had.
It is most interesting between 10-34 weeks I think, either side of that the baby is too small or too big to be that clear on the screen.
Hope that helps!