First of all big it's a worrying time and I'm sorry you are going through this

My experience with slow rising hcg levels did end in a mc when i was 8+6 but I was also bleeding which started before I had the hcg levels tested. When I went through my mc I scoured the Internet and there were so many positive stories after a slow start with hcg levels so there is always a chance everything will be fine and it is just early days. I will keep my fingers crossed for you