Maybe it was a chemical pregnancy if you are sure you are 12 weeks. That's the only explanation I have. At 12 weeks u should definitely be able to see the baby via ultrasound
It took three blood draws, one during, and one per week for two weeks after I had the miscarriage that the HcG level went to zero.
I had a "missed abortion" and it took a couple weeks for the natural miscarriage to start. Once it did, It took over 6 weeks after I miscarried before my HCG levels zeroed out.
Hope this is not the case for you, and that you are not as far along as previously thought. Good luck.
When u have a miscarriage U still come up positive as pregnancy it takes awhile for for it to be negative. That's what happened to me. The end of Last June i miscarried and n aug it still said pregnant and I wasn't