Is it normal for my HcG levels to be 690.5 and my ultrasound to only show a sac?

I recently had a positive pregnancy test and because I had a recent cyst and some abdominal pain the nurse scheduled me for an ultrasound. The cyst was gone, but I should have been 7 1/2 weeks and the ultrasound only showed a sac instead of a fetus with a heartbeat. They sent me for bloodwork and my HcG was 690.5. I haven't gotten the results back from the 48 hr repeat, but I was informed by a nurse that if the hormones don't at least double I will probably loose the baby and should consider terminating it now, which is not an option for me. When should I start to worry?

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