IVF pregnancy. Measuring behind. Enlarged yolk sac???

Finally pregnant after IVF. I should be 7 weeks 5 days today. At my 7 week ultrasound doc said baby is measuring at 6 weeks 3 days and that the yolk sac is slightly enlarged at 4.6mm he wants it under 4mm)

Heart rate was 125. He said that he’s a little concerned and that our chances for miscarriage are a little higher. Next appointment is on Thursday for an ultrasound at 8w1d.

Does anyone have experience with this? I feel so terrified that this will go wrong. 😭