Twins! But only one heartbeat!

Nicole
Hey guys!  I went in and had a scan done when I was 7 weeks 5 days according to my LMP. I had a scan done just because I've been so sick and I knew something was different. We found twins! Two sacs. Two yolk sacs. Baby A measured 6 weeks 6 days, 9 mm, heartbeat 155. Baby B was much harder to see, measured 5 mm and no heartbeat found. I'm hoping baby B just implanted later and when I go back for my follow up we find two strong healthy babies. No bleeding. No cramping. 
Anyone else have this experience? What was your outcome? TIA!