IVF 6 weeks 5 days first ultrasound all they could see what gestational sac

I had a positive pregnancy test after IVF and went in for my first ultrasound at 6 weeks and 5 days today. The doctor saw a gestational sac but almost no yolk sac or something really small, maybe even remnants of a yolk sac. I go back on Monday but he said he was very concerned, that I looked like I was a week earlier than I was and it could have meant miscarriage. Has anyone seen this and gone on to have a healthy baby or am I looking at miscarriage?

I’ve read sometimes there’s little or no yolk sac bc of a miscalculated pregnancy date but that’s impossible since we did IVF