@Background Pony #3F39
The “official” lore is that she works as a surrogate, and got her cutie mark in her early teens by accident.

@Background Pony #C293  
Well, somepony needs to maintain Equestria’s population. With so many lesbian couples, it’s up to those bold mares who get their cutie mark at the same time as their first positive pregnancy test.

@Background Pony #323E  
A better question: What would her parents say to that?

I always figured her special talent was either actual buns in ovens (Mr. and Mrs. Cake, you’ve got some competition!) or something to do with tending to newborn foals or pregnant moms, simply because you get cutie marks at an age where you shouldn’t be able to get pregnant at all. (Think of the CMC… and then remember they’re late bloomers and the only ones in their class without ‘em!) As for the name, her mom said “aw, you’re such a li’l bundle of joy - aha! We’ll call you Bundle Joy!”
 
That’s the most logical explanation, but of course, she’s obviously the product of someone’s pregnancy fetish, so her maker’s intent definitely wasn’t that she’d be a baker or obstetrician living a normal life that happens to involve having kids at one point. Nah, she’s always pregnant, because magic.

Your special talent is getting pregnant. …What do I even say to that?

@ze  
argreement this was my thought as well master, you show excellent logical skills, for a meatbag

Well, that must have been difficult.