Well, a prediction of "girl" is less a sure thing than a prediction of "boy" since it only means that she did not see a penis. Since an ultrasound does not produce high-resolution images in fine detail, it is possible to miss a penis or confuse an umbilical cord for one (my tech warned me that this happens rarely, but it does happen). Also, thirteen weeks is really early for making gender predictions. We happened to catch a "suspcious nub" in our sixteenth week peek, a nub which was likely to be a penis, but I knew better than to rush out and buy blue onesies.