An irregular gest sac as its known is not a good sign. As your doctor has mentioned,there may be chances of a miscarriage
. Its too early yet to confirm hence,the follow up usg was advised.
Please note that you are currently in a dicey stage. By 6-7 weeks, fetal cardiac activity should be present.If not,then you may have a missed abortion
. Follow your doctors advise and repeat the scan after a week.