thanks for your query.
By 8 weeks, if your periods have been regular, a proper yolk sac and embryo with heartbeat should be visualized.
Also, if the gestational sac
is more than 20 mm, without an embryo being seen, then the pregnancy
is a failed ( non viable ) one.
Causes can range from genetic causes, to infections, to hormonal
imbalance, auto immune causes, as well as trauma, radiation
, teratogenic drugs etc.
I think the prognosis for this pregnancy is not good, and the next scan would confirm this.