Thanks for your query.
The following possibilities exist :
1. Twin pregnancy
with one conception being delayed, and the second fetus would gradually be visible in further scan.
2. Twin pregnancy with one pregnancy being failed or missed, and the yolk sac
is the remnant, which would gradually disappear.
3. Rare finding of true double yolk sac. Mostly this is a harmless finding, but might be associated with other chromosomal abnormalities.
Further course of action for you :
1. Review ultrasound
to determine which of the possibilities is true.
2. Regular scans for knowing the progress of the live fetus.
3. Double marker at 12 weeks and quadruple marker test at 16 - 18 weeks.
4. Anomaly scan at 18 weeks.
5. Regular folic acid
supplementation and regular antenatal care.