CONGRATS! to you both
Actually Klash...it all depends upon the ultrasound technician who is performing the ultrasound. Some are more experienced than others. It also depends upon if the baby is cooperative during the ultrasound. Did you see the ultrasound? Any pics/video? Your wife is well beyond the date in which the genitalia are present/visible. This is the way I tell when I do ultrasounds....if you see three lil thin slices (lips) its gonna be a girl. Sometimes there is a slight protrusion in between the lips but this is the clitoris. If its a boy....a much larger protrusion is visible and his lil boys/testicles.
I'm also certain she will have another ultrasound before her delivery and again, ask to verify the sex.
Hope this helps...if not, just yell back at me.