Many women are diagnosed with a blighted ovum quite early on in their pregnancy at maybe 5 or 6 weeks. Although it is possible to tell if you have a blighted ovum at this stage, when an ultrasound is performed, it is also possible to be mistaken if the u/s is done really early.
For example, if it is performed before 6 weeks then you really have to be pretty definite about your date of conception to get an accurate diagnosis. If you are only 3/4 days out then this could change things a lot. If, say, you are working on your LMP date but you do not know your ovulation date, you could have ovulated a few days earlier or later than you thought, especially if you have a longer cycle.
This is the reason why the doctor will (or should) do two scans before confirming the diagnosis of the blighted ovum – the change in the size of the sac and/or the levels of hcg at this stage will be more of a give away to the fact that the pregnancy is not progressing as it should.
Therefore, if you are diagnosed with a blighted ovum before 6 weeks and offered a D&C, but not another scan, then it is worthwhile asking for a second scan to confirm diagnosis before taking any further action.