What a better day to pull off an office prank than Mother's Day right? Okay, so maybe it isn't the obvious choice for a perfectly timed prank, but if you're in the mood to embarass some coworkers, maybe Mother's Day will soon be your favorite holiday.
Because Mother's Day falls on a Sunday, most of you probably won't be in the office on the actual holiday. As such, you'll probably want to pull this off on the Friday before. What you'll want to do is buy a Mother's Day card that's for newly pregnant people and fill in the name of a female coworker you want to be one of the victims of your prank. The coworker needs to not be pregnant or have kids. Then, sign a bunch of your coworkers' names to the card and find another coworker to be the other victim. Tell this person that a bunch of your coworkers got together and signed a Mother's Day card for so-and-so (the initial victim).
Because that person isn't a mom, your coworker will be a little confused. Either they will feel really embarassed that they didn't know the person was pregnantand won't ask any questions or they will confront you about it. Either way, you want to come right out and basically shame them for not knowing the person was pregnant and that it was a pretty big deal and that everybody already congratulated her and everything.
Now you just sit back and wait for the fireworks. Obviously you'll not want to hand the supposed pregnant person the card because it has your name and a bunch of coworker's names all over it and would then be a dead giveaway. Instead, you just wait for the person you had sign the card to make a move to congratulate the no-quite-pregnant coworker. It may take a few days but you'll have planted a seed that will eventually grow into one heck of an embarassing conversation that will surely make some waves.
If you're worried about hurting the feelings too much of the female that you're pretending to be pregnant (because she may take it as looking fat or whatnot), then you can get the female in on the joke and have her just act incredibly upset when the person congratulates her on being pregnant.