I had the same type if miscarriage about 2 years ago with the ultrasound at 8 weeks. There is a VERY low chance that the doctor is wrong, but if you conceived later in the cycle than you're estimating, then you may not be far enough along to see anything yet. I waited and hoped my doctor was wrong, but after a week or so my symptoms all faded. I miscarried at 12 weeks and it was the most painful and horrible thing I have ever been through. If it were to happen again, I'd probably go for the D&C.
she could be right... you could have suppose to been pregnant but the embryo didnt produce a fetus so you would maybe let it miscarry or d&c.. an empty sac is a condition where the gestational sac develops normally, while the embryonic part of the pregnancy is either absent or stops growing very early. Other terms for this condition are blighted ovum and anembryonic pregnancy.