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
). Fetal heart beat usually appears by 8
. 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.