levels usually double every 48 hours in early pregnancy
and later on they take about 96 hours. Their level also depends on external hormone injections, and this can give spurious results. As hCG given externally gets excreted from the body within 10 days, a repeat test may be taken after this time. The sac should be visualized once the levels get to 1000 mIU/ml. Also, a collapsed sac indicates a missed abortion
and there is little hope that it could get normal. Anyway, you may wait until your next checkup to see what should be done. Good luck.