"even though studies are now reporting 100 percent accuracy in determining
fetal gender between 11 to 16 weeks of gestation, in the average clinic
situation, fetal gender identification is often not undertaken until at least
16 weeks, and is often not reliable until 20 to 24 weeks -- and this is when
the baby cooperates fully!
In a study by whitlow, published in ultrasound in obstetrics & gynecology, a sample of 524 women underwent a detailed assessment of fetal anatomy at 11 to 14 weeks of gestation by means of transabdominal sonography and transvaginal songraphy (26 percent). Fetal gender was identified and was confirmed at birth. The overall success of correctly assigning fetal gender increased with gestational age. At 11 weeks predictions were correct 46 percent of the time, at 12 weeks 75 percent, at 13 weeks 79 percent and at 14 weeks a 90 percent success rate was reached."
koala labs claim to determine fetal sex reliably as early as the tenth week of gestation.