blighted ovum

I was pregnant with twins back in February. One fetus was an ectopic and the other was a blighted ovum. I was told that this was very rare and that I am healthy and should go on to have a normal pregnancy. We tried to get pregnant again in June and were successful. I am being closely monitored because of what happened the last time. My HCG levels were rising appropriately and I went in for an ultrasound on Thursday at the end of my 5th week. They said to come back for another ultrasound because it was too early to see anything. I went yesterday which was the first day of my 6th week. They are saying that it could be another blighted ovum. They can see the sac, but no fetus. They also said that it appears that I am only 51/2 weeks pregnant. I have to wait until Friday to have another ultrasound. I am very scared and I am not sure what to think. Any thoughts, or similar situations would be helpful.