More times than not, if they tell you it's a girl, it's a girl. I had a 15 week scan and they told me it was a boy.. 22 weeks it was a girl. They told me it happens all the time and that most times the cord looks like a peepee and not a little girl. So it's an easy mistake.. a 3 D is more defined and it should stay a girl.
Had a friend went at 16 weeks and they said boy at 20 weeks the baby doctor said girl so be careful