Firstly, it should be 'sex' not 'gender', but that's not my issue.
Maybe I've lived in a bubble the past few years, but I've only just realized these existed. For those of ignorant ones like me, this is where a sonographer doesn't share the sex of the unborn child with the parents, but sends it off to a baker, who then makes a cake either with blue or pink sponge, covered with icing. It may get sent to a balloon supplier too who blows up a load of pink or blue balloons depending on the result.
So, the parents or parent, hosts a party at which guests are invited to share in the drama of finding out the baby-to-be's sex.... The cake is cut into and the assembled throng whoop and cheer as the sex is revealed, with many congratulations given.... The couple would then let loose the balloons (hitherto encased in some kind of sack to keep the suspense), gazing open mouthed in wonder as the news of their foetus' sex is carried off to the world on the four winds, providing an Instagram-tastic opportunity.....
AIBU to think this is kind of party is the height of a toe-curling, self-indulgent pretention that I hope never catches on (though I believe it already has the in the States).... or am I just being a grumpy, fun-hating grouch.