Could it be a wrong gender?

So me and my husband always wanted a girl, cause we already have one boy. The whole time jobs was 100% sure it's another boy, until the doctor said, well it looks like a girl to me. And I was looking at the screen and couldn't understand what I was looking at, then she says there is 3 little lines, I still couldn't understand what I was looking at, but couldn't hold my tears from happiness. But somehow I still not 100% convinced that is a girl. I'm 20 weeks. Did your doctor ever give you the wrong gender at 20 weeks anomaly scan???