The percentage of correct gender prediction is as follows:
11 Weeks - 50%
12 Weeks- 84%
13 Weeks- 90%
14 Weeks- 94%
"CONCLUSION:
Prenatal gender assignment by ultrasound has a high accuracy rate at 12 to 14 weeks. At 11 weeks there was an error rate of 50% and only 14/100 assigned correct male gender. In the male fetuses after 12 weeks, there was a significant increase in the angle of the genital tubercle from the horizontal. The accuracy of sex determination increased with gestation."
https://www.baby2see.com/gender/study_ultrasound.html
That being said, I see a Perinataologist on Tuesday where I would be 13 weeks. I will ask, but not go out buying things.
It takes a specially talented person to be able to differentiate at this state and most people will not even look if you are this early or tell you if they are doing an US anyway. If you're doing this to be certain, you have to wait until you are further along to get a definitive answer.
Like I said, I will ask, and if they tell me it'll be a nice "theory" until my actual genera revel at 20 weeks.