This artwork is based on the following short story by Kassaz:
https://www.fimfiction.net/story/526921/1/short-stories-with-pregnant-ponies/skintight-bodysuit

In the Carousel Boutique there were the beginnings of a one-pony fashion show. Tight clothes were in this season, and Rarity knew a bodysuit would knock the horseshoes off the competition, accentuating her curves and every nook and cranny. The only issue was wearing it.

Rarity loved her darling son, and watching him grow over the months had been delightful beyond words, but his presence in her belly made tight clothing infeasible. Sucking him in was impossible, lying on her back only crushed her organs like a corset, and magically enlarging the suit would be a bad idea once it shrunk back down around her.

She lay on her side with the suit hugging her rear and unable to stretch past the widest part of her barrel; she sighed. Turning to her belly in defeat, she told him “I’m going to dress you in the cutest outfits I can make.”