How many weeks were you when you found at the gender?

Katie💖 • Excited and scared to be a FTM, but ready for the new challenge!
I went at 17 and a half weeks. The doctor told me its 80% chance a girl. Has all the signs that it is, but could possible change at the next ultrasound.
How accurate was everybody's ultrasound before 20 weeks? I want to be positive that it's a girl!