Blighted Ovum is when the egg and the sperm meet and fertilise, just like in a normal pregnancy, but in this case only the gestational sac forms and the embryo itself does not develop. There may also be a fetal pole but essentially there will be no embryo. There are reasons for this to happen to your body, check out why this may happen here.
What you will see on the ultrasound scan is just a kind of ‘black hole’ which is the gestational sac. Unfortunately even with just this sac your body will think that you are pregnant and you will go on producing HCG and even making you feel like you are pregnant, including possibly having lots of pregnancy symptoms. However, as the embryo has not formed, the pregnancy is unlikely to continue any further than the first trimester, although with some pregnancies the body takes a while to actually start miscarrying.
A blighted ovum is created when there are chromosomal abnormalities in the fertilised egg and to prevent a malformed embryo, it does not develop any further, but the gestational sac does. As the embryo does not develop, this is also called an anembryonic pregnancy. A blighted ovum can only really be detected by an ultrasound scan as there are often no outward symptoms that this is the case.
Ultimately the body will reject the pregnancy and you will have a miscarriage but the point at which this happens is different for each woman and for some women the better option is to have medical intervention rather than waiting for nature to take it’s cause. If you suffer from a Blighted Ovum then you should be given the choice as to whether you miscarry naturally or whether you have some kind of medical intervention.