What Does My Vaginal Ultrasound Scan Report Indicate?

I had an hcg of 5800 and 48 hours later 7900. I had a vaginal ultrasound with the first blood work and it showed an 8mm gestational sac with no yolk or fetal pole. My doctor wants another hcg test 72 hours from the last one and is not overly concerned. I am concerned that the combination of slow rising hcg and nothing visible in the sac indicates a pending miscarriage. What are the odds here?