When should I schedule my gender determination ultrasound?

So I scheduled an appointment at a 3D/4D ultrasound place to find out the gender. My bf and I think the 3D ultrasounds look creepy though, lol, so we're just getting a regular black and white 2D one. The place guarantees determination at 17+ weeks. Right now my appointment is made for April 13th (over 2 weeks away) and I'll be 18weeks 3days. But I'm getting super impatient!! I'll be 17 weeks exactly one week from today (April 3rd) and I'm debating rescheduling the appointment. My only concern is that since it's a 2D ultrasound, it might be more difficult to see. I don't want to reschedule a week earlier and then be told that it's too early to tell. What should I do?!