I experienced bleeding and cramping early in this pregnancy and ended up with a clot in my uterus. In my case the lining in my uterus was extra thick due to fertility drugs and the placenta either tried to dig in deeper or separate and as a result I bled a lot. I had a ton of clots which totally was irritating my uterus and I could just tell that my uterus wanted to get rid of everything in there. I ended up having one large clot until about 12 weeks and I had ultrasounds monitoring the clot breaking down. The ER docs didn't know if I miscarried or if I was going too, and also said there was nothing I could do to prevent it and that I did nothing wrong to cause it (yeah, they were a BIG help). So I did what I knew to do to save my baby.
There are a couple reasons to miscarry: not enough hormonal support and then there is something wrong with the way the cells divided. I recommend you read Wise Woman Herbal for the Childbearing Year by Susun Weed as it has great herbal remedies to prevent miscarriage. I personally used Wild Yam and Wishgarden's Welcome Womb which is similar to the formula Susun recommends. The Welcome Womb treats it on a couple different levels, the wild yam to provide the progesterone support, the false unicorn root to help the placenta attach well and the black haw to stop the cramping. They all work to prevent a viable pregnancy from being expelled from the uterus. Vitamin E from a natural source helps glue the placenta back to the wall as well.
I also agree with the suggestion of having your progesterone checked, if it is too low you can get the progesterone suppositories. IMO if the HCG isn't doubling the proper way that is usually a sign that there is something wrong with the developing baby, but if it is just the progesterone then get some additional hormonal support. I felt that I just needed to do what I knew to do regarding herbs and I prayed a lot too. So that whatever happened bad or good, it was meant to be. I am now 27 weeks with a boy. I totally credit God, the herbs and the progesterone suppositories. Best Wishes to you!