possibly by 5 completed weeks gestational sac
will be visible in transvaginal scan and by 6 weeks in trans abdominal scan, by next week even heart beat of the fetus can be visualized,
you didn't mention your lmp or gestational age
anyhow if you crossed 6 weeks there is possibility for blighted ovum
anyhow as you are going to have scan soon, you can find out the actual situation.