Vanishing twin

Michelle
I went in for my first apt at 7 weeks (it was suppose to be at 10 but they kept rescheduling it). I found out I was pregnant with twins but one of them was only measuring at 5 1/2 weeks but both had heartbeats. I was told to expect to lose it so I went in 2 weeks later for another sonogram. One of my babies is growing and had a great heartbeat but as suspected the other one was actually a little smaller and no longer had a heartbeat. Has anyone had this happened and if so will the baby just disappear? My doctor doesn't seem to think my other baby is at risk but I can't help but worry.