The video of this April Fool's Day prank, played by students at Aquinas College on their Macroeconomics professor, now has over 25 millions views on YouTube, which has to make it one of the most popular April Fool pranks this year (if not the most popular). It's nice to see that a low-budget prank by amateurs still can overshadow all the April Fool marketing efforts of the advertising professionals.
The premise of the prank is that a female student receives a call on her cell phone during class. The professor has a rule that if a student has failed to turn their phone off, and it rings during class, they have to answer it in front of everyone. So the student proceeds to take the call, only to learn that it's from the "pregnancy resource center" informing her that she's pregnant. The look of horror on the professor's face as he hears this, and begins to imagine the repercussions of having forced the student to share this news with the class, is classic.
Fake pregnancy announcements are actually a fairly common prank on April Fool's Day. The typical set-up is that female employees will tell their boss on April 1 that they're pregnant and have to take time off. The prank works best if multiple female employees make the same announcement, leaving the boss to imagine the prospect of losing half his staff. I've recorded an example of this from 1963 in the April Fool Archive