Hello...

I was told at 16 weeks that I was having a boy. Even though this is # 1 for me, I was a bit disappointed as I wanted a girl....but I got over it by the time our NT scan came around (I was 20 weeks 5 days at this point). So during the u/s I asked the sonographer to confirm it was a boy and he said "It is not a boy, it is a little girl!" and proceeded to point out the entrance to the vagina an the "3 white lines".
I was in shock for about a week!
BUT.... Although I know you should trust the later scan, I am still not 100% convinced and will have to wait and see what pops out. So I don't think you can ever be 100% sure unless you get one of those 3/4D scans later in your pg.
Good luck and try to remember all babies are miraces, no matter what is between their legs