My first miscarriage was diagnosed as a blighted ovum. I was sick, since my hormones were increasing and my body was displaying all the "normal" signs of pregnancy, even after I knew the sac was empty.
I found out at my first sonogram, scheduled for 7 weeks, 6 days. At that stage, the sac was only measuring 5 weeks, 5 days and was "abnormally shaped." My doc did not point out how it was abnormal, but seeing my son's sac, I can tell that it was flatter and smaller in general (my son's first sonogram was at 8 weeks, 1 day, so they were comparable). My body was holding on though, which is apparently a blighted ovum symptom. I did miscarry naturally a week and a half later. But part of that was because I found out kinda late that the sac was empty.
I am really, really sorry you have to go through this. It really sucks to have your hopes up and then have them dashed in that way. I will say, expect that even after you miscarry or have a d & c (I don't know how many people need it for a blighted ovum, but I know that lots of folks have a hard time passing the pregnancy, since your body can be slow to catch on), you might be sick for a while afterward. I still felt pregnant for a week or so, when my hormones finally fell (I did with my second miscarriage too).
Ask as many questions as you want, we have all been through something similar and as long as you are not being malicious (like rubbing a pregnancy in our faces), we are here to answer as many questions as you have.