I had an ultrasound today at five weeks (I believe to the day). However, my gestational sac was only measuring an average of about 4mm, with the longest dimension being 6.2mm. The ultrasound sonographer didn't appear concerned, and said that there is a chemical reaction between my sac and my uterus. But from what I've read, 4mm is quite small for 5 weeks? The very earliest I think I could be is 4 weeks and 6 days.
What experiences have people had with gestational sac scans at the ~5 week mark?