I found out through an US myself that I was pregnant with twins. Their gestation was farther apart than what yours are. We were told from the beginning that it may be a vanishing twin. At about 7 weeks, we saw HBs in both babies so we thought we were safe, even though one was slower HB and still smaller. We were cautioned, still, of the vanishing twin. Never did we think after seeing the HBs, we would lose one. However, we did. We were crushed. But we now have a very healthy 2 year old daughter. She is a fighter. You need to remain positive about this, and remember that you have at least one to live for. Most women do not have US's that early, so do not know they were even having twins. We thought we were miscarrying, so that is why we had such an early US. Remain positive, and know that you also may never pass anything. With a vanishing twin, your body actually absorbs the sac. It is sad, none the less. But stay strong, for you have no reason to think you are losing the other baby.
Good luck and stay strong.
Phoebe Grace 6-22-11; 37.5" and 26.2# at 2 years old! She is my wild child!
Me & Geoff, 40; DD Phoebe, our June Bug