Finding Out The Gender...

When is the EARLIEST gender can be predicted? I'm only 12wks and won't have another ultrasound until I'm about 20-24 weeks :( I CANNOT WAIT THAT LONG! Lol. However, I'm willing to pay for an ultrasound before my doc gives me one... Just want to make sure when I do it, it's a good enough time to find out the gender! Sooo ladies, how soon did you know???