HCG not doubling! Possible miscarry? Help!!

Lulu
At 6 weeks my Hcg was 3580, 72 hours later it was 5240. I had an ultrasound 4days after and saw a heartbeat (7 weeks) at 126 bpm. I am not feeling very confident that this pregnancy will be viable due to the low rise in Hcg. I am not bleeding etc but I miscarried at 7weeks in Dec and I wasn't bleeding then either. Anyone else have a similar story????