Can the gender be wrong !

Moka
Hello ladies !
I do my anatomy scan at 19 weeks !
My doctor told me that my fraternal twins are both girls.  I was having a very strong feeling that they are boy/girl.
I already had 4 year old daughter and will be very happy if one of the twins is a boy !
Is there any chance that the doctor can mistake the gender specially with twins ! Any stories ?!
Or just wanting a boy very badly making me feeling like that !
Another issue, I want to do 4D US what is the best week for twins to do it ?!
Thank you