HCG levels double until about around the time of the second trimester (week 14).
How is your beta increasing? I've been reading a lot on the internet about beta's today trying to figure mine out and after a certain number of weeks the betas no longer double every 48 hours, they can take up to 4 days or more to double.
For example, I don't know how far along I am, but on Monday my beta HCG was 59,000 and on Wednesday it was only 79,000. The OB's aren't worried about me because I am not cramping, I am not spotting, I am approximately 6-8 weeks along and betas tend to slow down the higher they are and the further along you are.
I am worried, but I am a worrier. You may be just fine. The chances of a miscarriage after seeing a heartbeat drop to roughly 5%. (hug)
Usually, your hcg will stop doubling when there is something wrong with the baby. At this early stage of pregnancy, it most likely is due to chromosome problems with the embryo. I've had 3 m/c and with the last one, a heartbeat was detected at my 6 week u/s. I unfortunately I lost that baby but I know that I have a chromosome problem and my chances of m/c are higher than the average woman. After 2 m/c, you probably won't have fertility testing. Usually, they wait for 3 m/c. Best of luck to you and I hope that you find out what's causing these issues and you find a solution.