Unlike other entries, this story by Kassaz was based on the artwork:
https://www.fimfiction.net/story/526921/24/short-stories-with-pregnant-ponies/a-mundane-victory

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Rarity smiled, putting the finishing touches on her new dress. Preparing for foals was busy—so many little clothes to sew—which had delayed the outfit for weeks. She’d be the talk of tonight’s party wearing it.

Rarity stepped into the dress, pulling it around her hindquarters magically. She noticed one problem: It didn’t seem to fit. She tugged until it threatened to rip, and changed her strategy. Rarity found the dress no easier to wear lying on her side or wiggling on her back, however, and huffed.

She’d taken her measurements perfectly, and chewed her lip wondering what was wrong.

“Oh! I took my measurements for this dress weeks ago.” She frowned. “I may have grown just a little since then.”

Magic dragged off and dropped the outfit. She had a beautiful maternity dress perfectly sized, for an earlier pregnancy. “Well, I’ll just have to wear this next time, I suppose.”