Measuring a week behind, heart rate only 77

I'm not sure what to think. Please help. My FET was 1/11. My first beta was 124 on 1/21 and then 500 on 1/25. Last week I had a vaginal ultrasound and there was no fetal pole, I was measuring a week behind at 5 weeks 3 days. This week the fetal pole is there and I'm measuring 6 weeks 3 days. They said the yolk sac is larger than normal so that could mean chromosomal abnormalities.

I'm supposed to be 7 weeks. Has anyone had slow growth in the beginning and turned out to have a healthy baby? I'm so nervous! I've already been through 2 miscarriages and would be devastated to go through it again.