Ultrasound to determine gender

Ka

Hello! I am debating on when to make the appointment to determine gender via ultrasound. My Dr. won’t do another ultrasound until 20 weeks and I just don’t think I can hold out that long. I am going through an independent company to do it. My question is whether making the appointment for the day I hit 16 weeks is okay or if I should wait until closer to 17 weeks? I’m worried if right at 16 weeks will be too early to tell