Short of genetic sampling, an ultrasound is probably the most reliable method for gender prediction. One of the biggest factors in how reliable an ultrasound will be for gender prediction is how early it is used. If an ultrasound is used for gender prediction too soon, it is very likely that the result may either not be very accurate, or at the very least may not be very clear.
How soon an ultrasound can be used for gender prediction also depends on other factors, as well. The position of the baby plays a role in gender prediction. The thickness of the abdominal wall will also determine how soon an ultrasound can be used for gender prediction, as a thicker abdominal wall can delay gender prediction for sometimes as many as 10 weeks.
Generally speaking, with all of the other factors being equal, an ultrasound can be used for gender prediction by around the 20th week of pregnancy. Having said this, the fact of the matter is that an ultrasound can be used for gender prediction much earlier, sometimes as early as 11 or 12 weeks if the latest ultrasound equipment is being used. Of course, the later that the ultrasound is used for gender prediction, the more accurate the results will be. The optimal time frame, in terms of the reliability of gender prediction when an ultrasound is used, is right around the 20 week mark.
It is also important to keep in mind, when an ultrasound is used for gender prediction, that the skill level as well as the experience of the person who is operating the ultrasound equipment plays a role in both whether or not gender can be predicted, and whether or not that gender prediction will be accurate. Also, some ultrasound technicians may predict that the baby’s gender is female when, rather than actually seeing the female genitals, they cannot see a penis on the ultrasound. When your ultrasound technician predicts the gender to be female, it is worth asking whether she actually sees the female genitals, or whether her prediction is based on the absence of a penis only.