M.B.
S.,
Ultrasounds are not 100%, it all depends on how cooperative the fetus wants to be. But they are way better than 30 years ago, if they were around then. For us, I didn't want to know with my son, but did with my daughter. She was cooperative enough the the tech got a good look at her parts and with certainty said a girl. She even pointed out to us, and printed a picture of the "girl parts" which to this day I don't see. I even asked her several times if she was sure it was a girl, and without hesitation she said: girl.
I'd talk to the doc and maybe look over the pix with him/her if you have some doubts. Maybe at a later date get another ultrasound, if insurance covers it.
Hope this helps,
Melissa