Hi, I just went through the same thing last month, but it didn't work out for us. Here's an article I found on it though:
Outcome of First-Trimester Pregnancies with Slow Embryonic Heart Rate at 6-7 Weeks Gestation and Normal Heart Rate by 8 Weeks at US -- Doubilet and Benson 236 (2): 643 -- Radiology
When we went back for the second u/s, the HB was slow. Then when we went back for the 3rd u/s, it was stopped. Probably not what you wanted to hear, but like the article says, it could pick up.
If it doesn't work out, you can do a D&C and then get genetic testing like we did. (Insurance should cover) We found out the baby had an extra Chromosome 16 which is why the baby didn't grow and the HB was slow.
I really hope your baby's HB catches up.