Could the ultrasound be wrong about my baby's sex?

Maybe: There is absolutely a chance that an ultrasound could show the incorrect gender. Even with the best equipment and the most experienced ultrasound technologist, errors can be made. An ultrasound is most accurate in determining the gender at approximately 20 weeks gestation. Occasionally, the ultrasound technologist confuses the umbilical cord with a penis and declares the baby boy. The more common error is made when the ultrasound technologist fails to identify a penis and determines that the baby is a girl. Sometimes if the baby's thighs are crossed, determining the genital anatomy is nearly impossible. However, if an experienced ultrasound technologist gets a good view of the anatomy they will be a right nine times out of ten.

Yes and no!: Depending on how early an ultrasound is done on a pregnant women, the more inaccurate the results on gendering will be. If done later, ie. After six months, they are very accurate.