The Cutie-Mark Crusaders look into getting a particularly marely kind of cutie-mark, but Apple Bloom notices something else about herself in the process.
Reproduction, it's a perfectly valid thing that makes total sense. When a mare and stallion love each other very much... Blah... Blah... Blah. Here's a foal. That's how it's supposed to work. Except for when it does not.
Set after "We Regret to Inform You" :D Pinkie Pie becomes a mother of six children, not including Pound and Pumpkin Pie-'cake'. Pinkie and Cheese then marry and have kids. The Twins now have to deal with sextuplet additions, making a huge family! :)