Question for Team Green...how to keep it a surprise?

HeyJune

We found out our baby's sex at 18 weeks with our last pregnancy, and we've decided to wait until delivery day this time around. 

I'll be having a repeat c-section. I have a fear that someone along the way, maybe a nurse, ultrasound tech, or the doctor will accidentally give away the baby's sex. There just seem to be a lot of instances where it could slip unintentionally. I've heard of this happening so many times to others that I'm becoming more and more concerned about it. I guess it is because most people find out, so a lot of healthcare providers assume you know.

Any ideas on how to make sure it is kept a secret? Could I ask that my dr put a note in my chart? Do they make a note for anyone performing ultrasounds after the 20 week one? I'd rather find out at 20 weeks with my DH present than in an exam room by myself at a random appointment (or 15 minutes before delivery when someone accidentally ruins the surprise). Advice from those who've been team green before?