slow rising hcg levels

I found out I was pregnant about 2 weeks ago. I am high risk because of a ruptured tubular pregnancy that happened 6 years ago. I had a vaginal ultrasound last tues, and we couldn't see anything. My last period was on Sept. 6, but I am 30-33 day cycle person. I had my hcg levels taken and they were 680something...two days later they only rose to 800something. I am going tomarrow for another blood hcg test. If they don't rise again I am schedueled for a dnc on Monday. Is there a chance that I was too early to see anything on the ultrasound, and that my I am just a slow riser of hcg levels? I don't want to have a dnc if there is the slightest chance...is there any hope for this???