I found this info when I first learned my doc was giving "80 per cent certainty" predictions at 12 weeks. The conclusion is, it's very accurate even at 12 weeks, if the practitioner is using the right techniques - but of course in some cases they'll be mistaken.
http://www.ob-ultrasound.net/genderfaq.html
On the subject of "early diagnosis of fetal gender", a recent article by B J Whitlow et al, entitled "First trimester diagnosis of gender" which appeared in the Journal: Ultrasound in Obstetrics and Gynecology 1999; 13:301–304 addressed this issue.
This study addresses the question of first trimester diagnosis of fetal gender by assessing the outcome of 447 women who had normal transabdominal ultrasound sometimes complemented with transvaginal ultrasound (26%) when the genitals were not clearly visible. Patients were scanned between 11 and 14 weeks and were referred for assessment of fetal anatomy.
Results showed that the overall success rate of gender identification between 12 and 14 weeks was 80% which is less than invasive karyotyping. In the transverse plane, male fetuses showed a dome shaped genital swelling with a cephalic-directed phallus and females had three or four parallel lines representing the labia. The gender of the fetus did not affect the operator’s ability to assign gender and when assignment was possible gender did not affect the accuracy. The ability to assign gender increased with increasing gestational age.
Another article, Determination of gender - 10 and 14 weeks by Z Efrat et al which appeared in the Journal: Ultrasound in Obstetrics and Gynecology 1999; 13:305–30, reported:
The 172 women were all scanned transabdominally.
....
The phallus was vertical/cranially directed in males and caudally directed in females. At 12 weeks the accuracy of gender assignment was 98.7% and at 13 weeks the accuracy was 100%. There was incorrect assignment of males as females in 3% at 12 weeks and no females assigned as males after 12 weeks.