Gender scan at private ultrasound office

We are going tonight to find out the gender! My doctors office won't do a gender scan until 20 weeks, so we are going to a private place tonight, so worth the money to find out 5 weeks earlier then the doctors office! Anyone else gone to a private ultrasound place to find out earlier? If so, do you tell your doctor that you already know at the next appt or pretend it's a surprise?!