Measuring behind??

Hey y’all! So my periods were generally within the first week of the month, the date could change but it was always within the first week. So going by the start of last period (which should be 02/01/23) I should be 7w1d.

We just had a dating scan today and I’m measuring 6w2d.. I’m a little confused why this would be.. I don’t have the images yet (should be uploaded in a couple hours) so I can’t show the sizing but has anyone had this??

Also I don’t know if this is helpful info but heartbeat was 138bpm