@Billie2 every mm equates to 1 day and with everything being so small, it would be hard to measure. I'd say that's why they don't do official dating scans this early. Even if it is 4 days behind, I wouldn't think that's a definite no for viability, not that I'm the expert. I only got an abdominal scan, part of me thought the following day that it would have been good to get an internal one as at 6wks, we would have been able to look for a hb. The main purpose of the early scans are just to check that everything is growing on track and most importantly in my case, was the location of the sac. I guess the scan served it's purpose.