Were your levels 600 or had they dropped by 600? If they were only 600 and you are certain of your dates, those numbers are just too low. However, anytime after about eight weeks, your levels are supposed to taper off and then begin to decline.
This may very well be a miscarriage but I'm glad you are having that next ultrasound to be certain.
To read my own Misdiagnosed Miscarriage stories:
my blighted ovum scare: http://www.squidoo.com/misdiagnosedblightedovum
Told I was miscarrying at 12 weeks: http://www.squidoo.com/firstmisdiagnosedmiscarriage
If you are being diagnosed with a blighted ovum after IVF, please take a look at this article I put together: http://www.squidoo.com/IVFMisdiagnosedMiscarriage
and why I just believe a blighted ovum should NOT be diagnosed before nine weeks at the earliest:
This post is for informational purposes only. This information is not intended to replace information given by your physician but rather intended to supplement that information.