Ultrasound growth abdominal vs transvagin

Hi ladies

I am slightly concerned. I had a final visit with my fertility specialist 2 days ago and the transvaginal ultrasound measured 15/16mm and 8w1d which is spot on from when I ovulated and everything looked perfect.

Today (2 days later) I have the first appointment with my OB. He wanted to have a quick look with the abdominal ultrasound and said the heartbeat looked very strong and that I measured 14/15mm and 7w5d - it was a very quick scan and way less clear on the screen then the transvaginal.

How would the date go back 3 days when 2 days had gone forward? Shouldn’t it now be 8w3d? Is it because the abdominal ultrasound isn’t as good as measuring this early or because the baby has shrunk? - which would be odd with a strong heartbeat. Seeking some reassurance here I’m super nervous the baby has stopped growing.

Thankyou!