I am sorry you are going through so many tensions regarding this pregnancy. Normally a gestational sac with a yolk sac appears at about 5 weeks (calculated from your last missed period date) of pregnancy and can be well visualized by 6 weeks on a transvaginal (through a vagina) ultrasound (so it can be missed at 5 weeks). Fetal heart beat usually appears by 8-9 weeks. This is why it is best to repeat the scan a week or two later.
At times the sac appears but there is no baby inside. This is called blighted ovum. This is confirmed if an ultrasound through vagina reveals a sac with gestational sac diameter greater than 13 mm and no yolk sac, or diameter more than 18 mm with no embryo. Follow-up ultrasound is done after 10 days to verify diagnosis. Normally it is wise to wait until 10-11 weeks of pregnancy for a confirmed diagnosis of blighted ovum or fetal death. Meanwhile monitor the HCG levels at 48-96 hours intervals. If they rise and almost double in 48-96 hours, then chances of pregnancy carrying forward are high. Please discuss with your gynecologist. Take care!
Thank You very much for the help. That made it a lot clearer. We have an ultrasound and an appointment tomorrow. Crossing my fingers for a yolk sac and fetal pole.