Gestational sack measuring 4 days behind CRL

I went in for my first ultra sound today at what I believe is 6w4d.

It looks like they measured the gestational sack at 1.5cm and noted that is the average diameter for 6w2d. The the crown-rump length measured at 8mm and they noted that is the average length for 6w6d. They did see a yolk sack and a heartbeat with a rate of 149 bpm.

I'm still waiting to hear back from my ob and obviously a bit anxious. Anyone have any insights or personal stories they'd be willing to share?