Slow rise hcg & heartbeat at 6 weeks

I’m not sure what is happening….

My hcg levels were not rising as they should (23/9 - 1700, 27/9 - 3300, 30/9 - 4200).

I went in for an ultrasound expecting a miscarriage but we heard a heartbeat.

I’m measuring 6w1d and I know I should be closer to 7 weeks.

My dr is going to monitor me with regular scans to see how it progresses.

Has this happened to anyone else?

My partner and I were at peace with a possible miscarriage (it would be our second one this year)