i have had two pregnancies diagnosed as Anembreyonic. As the PP said, the old fashioned term is a blighted ovum.
In simple terms it means that the embryo has died and been reabsorbed back into your body but the gestational sac has continued to grow (usually at a reduced rate). You will usually continue to get pregnancy symptoms though you may have noticed that they are less intense. Depending on when it is diagnosed it may be recommended you have a D&C to end it.
It is a horrible diagnosis and if you have been given it my heart goes out to you. A lot of old school medical "professionals" tend to treat it like your baby never existed. I had one describe it to me as like a phantom pregnancy, not a real one. I think she thought she was making it easier on me but i nearly decked her. You baby was real and so is your grief.
Blighted ovum is the major diagnosis for early pregnancy loss and is usually caused by chromosonal issues in the embryo. It is rare for a person to have more than one but it does happen. For me it turned out that i had a blood clotting issue (lupus anticoagulant antibodies) which was causing my embryos to be starved of blood supply.