Twins: Only 1 Yolk Sac

Desshalay
I went to the ER because I was having a little bleeding only when I wiped, but still was scared. Went In for an U/S and Doc told us he only seen one yolk sac! All the tests came back negative and he's also said it's too early to tell if both will make it, he was pretty vague & I'm worrying at this point. Has this happened to anyone else??? Oh I was 5w1d here go back for another 12/10