DO THOSE GENDER TESTS REALLY WORK? 20 points best answer!!?

I'm 13 and a half weeks pregnant. I wanted to take the gender test. Its in the same section has a pregnancy test and I want to know the sex of my child. I can't wait cuz my doc said he will tell me when I'm 20 weeks but I want to know already! They are pricey about 40 bucks and I think its a urine test, soo I want to know if it really works!