Beats me. Probably they wait until the stroke of midnight and pull their babys' diapers down.
Good gravy, that could be exactly it.
According to my Google results, this event occurs mid-pregnancy, following an ultrasound, at 20 weeks or so, with clear images. The party resembles a baby shower in that it features silly games.
Golly, what I missed ...
Can’t tell if you two are just being facetious, so I will define this stupid fad for you. At a “gender-reveal” the expectant mother lets her family and friends know what the sex of her unborn baby is.