Thanks for posting your query at DoctorSpring.com
Have you always been regular with your cycles ?
HCG levels are not relevant clinically once the fetal heartbeat appears.
Low fetal heart rate is associated with spontaneous miscarriage, abnormalities in the baby etc.
However do not read much into it.
Just get a repeat scan done, if the heart rate picks up and comes to the normal range, there is nothing to worry.
I hope you are taking folic acid and progesterone at the very least as medication.
Rest, avoiding stress, intercourse etc are also important.
All the best
Please feel free to discuss further.