IVF measuring behind

I had my second ultrasound this morning at 6w2d and we saw gestational sac, yolk sac, fetal pole and flicker of heartbeat! I thought these were all great things! But then dr came in to discuss and said that I’m only measuring 5w5d and that I’m about 4/5 days behind. She said she was “cautiously optimistic” and that when we go in next week if there is no heartbeat they’ll deem it as a miscarriage.

I’m terrified and have myself so worried. I thought what was saw was exactly what they were hoping to see but now I don’t think that at all and she has me concerned!

Has anyone had this happen and everything turn out ok?