Measuring a week behind

Crystal

I'm worried as I went for an ultrasound two days ago and was measuring 5w4d but thought I was a week further. I spoke to my doctor today and he said the ultrasound report said to come back for another scan in 10 to 14 days to see how things are going. He also said that there is two things that are happening either I ovulated late or it's not a viable pregnancy I'm so scared