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
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.
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.
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