IVF pregnancy ultrasound measuring 3 days behind

Has any had this problem, with the ultrasound gestational age being different from the technical fet date? Since we are pregnant from a FET, the date is 3 days off.

8 years ago with my daughter when I was pregnant I went in thinking baby was 9 weeks but baby only measured 7 weeks. Then when I had her she was 2 weeks overdue doctor always said 1 week off on dates plus or minus is no big deal. But then I just went into the doctor for this current pregnancy The ultrasound measured me at 6 weeks 5 days. But by IVF I would technically be 7 weeks 1 day so only 3 days off. The doctor said I'm cautiously optimistic which of course scared me and caused me to cry a few times the heart rate was also at 108. Because they know the exact day for IVF they think I should be exactly that far along but some babies implant early or late I thought. I looked up online and its very common for IVF pregnancies to be a few days off or a small baby at first. Also the heart rate for 6-7 weeks is 90-110 so at 108 I'd be fine. I go back in next week for another ultrasound but the doctor has me worried until then. Anyone have this happen to them?