How do expectant parents not already know the gender of their baby-to-be at gender reveal parties?

So there's a video popping up on "popular" across multiple subreddits: it's a compilation of parents-to-be who are disappointed to see pink, and learn that their baby-to-be will be a girl, at their gender reveal parties.

I am genuinely curious of the logistics of this: how does the expectant mother (or father for that matter) not know the gender? Does the OBGYN write down a message in a sealed envelope to be passed to a gender reveal party planner? Does a family member come to an ultrasound appointment and have it whispered into their ear?

I always assumed the father and mother both knew and the "surprise" was for the guests, but seeing the expectant parents surprised and baffled has be equally baffled.

This is one example, I'm making it a non-participant link.