its more likely a healthy sac if it is like a perfect circle, luckily for the lady above she carried to term the second time around but they usually prepare you for a miscarriage. did you see baby in the scan at 8 weeks? if not, it sounds like a blighted ovum. sadly its a waiting game. good luck, only a few weeks to go to 12 weeks!
is this for your personally? i had an irregular shaped sac with my 2nd and 4th pregnancies...my 2nd i lost at 8 weeks, diagnosed as blighted ovum and had a d&c, my 4th pregnancy it was diagnosed at 6 weeks as mis shapen and bigger than it should be for my gestation and was offered a d&c again, i refused and now have a healthy 6 year old daughter.