8 weeks measuring 6w3d

Hello! I went for my first ultrasound on 12/3 for pain in my side that ended up being a cyst. They measured the sac and said I was about 5 weeks 5 days along.

I just went in for what I thought was my 8 week ultrasound and I was only measuring 6 weeks 3 days. There was a fetus, and it had a heartbeat of 129bpm.

Based on my date of conception I should be 7 weeks 5 days, so not sure why the baby is only measuring 6 weeks 3 days. Drs didn’t not have a whole lot of answers, but I have to go back in two weeks to be rescanned.

Has this happened to anyone before? Have you gone on to have a healthy pregnancy, or is this eventually going to turn into a miscarriage?