you should not wait, you should go and see another doctor as soon as you can and get another scan.
i hope this does not distress you by reading this, but when a baby stops growing it starts shrinking at the same rate it would have grown. by 12 weeks you should be able to see the baby by ultrasound. my sac was measuring just over 8 weeks at my 12 week scan, the lady was very experienced ( i know her through my job) and did a very thorough internal, and saw the placenta was hydropic and the baby was tiny, measuring a few millimeters, with no heartbeat. you will have hcg in your system as it is produced by the placenta which is probably still there and still producing it. or as others have said it could be a blighted ovum, either way, you should get another opinion and another scan.
good luck and please let us know how you get on.