Missed Miscarriages?

Hi ladies,
I had never heard of a missed miscarriage before until I joined this app. I am 14w2d pregnant with what my midwife says is a picture perfect pregnancy. However I can't help but worry about missed miscarriages. How common are they? I am going to have an elective ultrasound done for gender in 2 weeks and my worst fear is that something would be wrong. I did just have an apt last week and she said the heartbeat was perfect and strong. I feel like I should have nothing to be worrying over but I think I'm letting this get to my head? Any advice is appreciated so I can stop worrying which is best for the baby.