How correct is an early gender ultrasound?

I was at my doctor’s for a 15 weeks visit to check the heart beat and she couldn’t detect it so she sent me down to ultrasound to have a look (thank god nothing was wrong the baby was just all the way backed into the back of the uterus pushed up against the placenta). Anyways the ultrasound tech asked if she could tell the sex would I want to know I impatiently and eagerly said yes! To my disappointment she said she was very positive the baby has a penis. I really want a girl! So please cam someone anyone tell me how accurate it is at this early of development?