Change of due date

Originally due April 5 today had an ultrasound and they changed it to April 12. Baby did have a heartbeat of 169.

I don’t understand how I could be measuring a week earlier though because I found out I was pregnant at like 3.5 weeks so then I really found out at 2.5??

I asked and they said not to worry but I’ve had a late miscarriage before (although that baby never had a strong heart beat).

Does anyone have any experience with this? It’s hard not to be a little worried when you’ve miscarried a few times before.