No, I had a dating scan at 5+6. We saw and heard the heartbeat, and saw the blobby floating around. It was pretty amazing.
Just had my 2nd scan at 9 wks, and baby grew textbook, to the day. So, obviously, the first time was a pretty good date!
They're not always 100% accurate, but judging by heart rate (it starts at around 80-103 bpm at 5 wks, climbing up to 140-190 at wk 9, then it levels off and goes down to 120-160 bpm), the crown to rump ratio, and doing thermals, they can tell what bones are forming, how far advanced the spine is, etc. and that allows for a pretty accurate date.
Good luck!!! My 6 wk was cool, because it was just a blob, and my 9 wk, it looked like a baby! So, that was neat. You'll love every minute, mama!