HCG levels low but suppose to be 8 weeks

I went to the hospital for some spotting. Not very much. I only had it twice when I wiped & haven’t had any since. It’s going on 5 hours now. No pain either. The hospital did a beta check and it was about 18595. Which would put me at about 5-6 weeks. But based off my period I would be 8 weeks & 2 days. The hospital did a abdominal ultrasound, although it was hard to see baby, it looked like it was measuring 7 weeks & heartbeat was detected. I was labeled as “threatened miscarriage” has anyone dealt with this & ended up having a healthy pregnancy?