This comic is based on the following short story by Kassaz:
https://www.fimfiction.net/story/526921/12/short-stories-with-pregnant-ponies/leftover

“So, uh, is there any cake left?” Rainbow Dash looked around the room. On the table, where an entire triple-story cake had been minutes earlier, were only crumbs.

“It sure doesn’t look like it. Seems like all the cake’s right here.” Applejack poked Pinkie Pie, lying on her side asleep on the floor, or rather she poked the pink pony’s protrusion that was about the right size for an entire cake. Pinkie was snoring, very, very loudly.

“We left the room for just a few minutes!” Rainbow complained.

“Um, I found one piece, actually.” Fluttershy brought over a slice somehow spared, from a different table.

“So now the question is who gets it.” Rarity was to the point. “I’m watching my figure, so I’ll be fine without.”

When the four looked back at the slice, it was gone. Pinkie hadn’t moved.

“Well, it was her foal shower anyway.” Fluttershy mumbled.