Kassaz commissioned this artwork and wrote this short story to go along with it, also available in this anthology:
https://www.fimfiction.net/story/569242/1/tiny-stories-with-huge-pregnant-ponies/burdensome-blessing

A group of unicorns is known as a blessing. Rarity had been told earlier in the year that she was blessed with fertility. She felt anything but blessed, with an entire blessing being gestated inside her. She was tired, and her back ached; so did her hooves. She was now wider than she was tall, and even that couldn’t hide that her butt was bigger. She had recently taken to sleeping while standing, and at least three of them were always kicking. What little work she’d been able to finish lately had been for other ponies; she wasn’t even going to try to cover herself up, she knew it would just be an expensive tent over herself. All she could wear from her old wardrobe were her bras, and just barely. It looked like she had a second butt, a very sensitive, constantly leaking second butt. That was another reason for her not to look in the mirror, besides the fact that she couldn’t fit all of herself in its reflection anymore.

Rarity huffed. A thought crossed her mind as she slowly made her way to the kitchen. She was so thankful for her magic, more than she’d ever been in her life before. She didn’t know how Mrs. Cake could do it. The second thought that crossed her mind, while she magically opened a cabinet and pulled down a plate and glass, was how thankful she was for civilization. She had no hope whatsoever of surviving in the wild like this, as prehistoric ponies supposedly had in the times before Celestia and even before the times of the three tribes. Magic and civilization, those were certainly nice. Prehistoric pony had been adapted to the wilderness, she’d also not so coincidentally learned they had been prone to singleton pregnancies almost exclusively. She didn’t even have to turn her head, she merely had to cut her eyes to the side, to see what had once been her bulging side that was now a bulging front. Magic and civilization, those had certainly made ponies smaller, and yet bigger at the same time. She sighed.

She arranged a salad on her plate, knowing that wouldn’t be enough. She opened her refrigerator, added some leftover hay fries to the salad, and forced a large slice of cake onto the plate too. Somewhere deep in her body, but really where it had always been, she heard her stomach rumbling. She sighed again.

-

After eating far too much, Rarity was still a little peckish, but decided instead to return to her showroom. Few outfits were on display, with her output reduced and ponies buying her stock to help support her growing family. One caught her eye, an item that wasn’t for sale, a cute little red dress she’d worn many times before. She hovered it over to herself, and thought about how it wouldn’t even begin to fit her now. She wanted to put it back, but that was a little sad, so she instead made some mental notes about where she could adjust it to make it wearable again as soon as she’d parted from her blessing and recovered. She was certain to shrink her barrel massively, but her rear would probably take a while longer. Only after imagining herself wearing the dress around her husband and many children did she hover it back to its place.

Her hooves were sore, more sore than usual. Leaning her butt against the wall would provide only minor comfort, but if she sat down she’d better be prepared to not get back up for a while. Her chaise lounge was in view, and she just may be able to shove herself onto it. She waddled on over, set her forehooves over on it, and grunted. She rather uselessly picked up her left hindleg out of instinct, even though she knew she clearly lacked the mobility to do anything except shift her weight around slightly. “Did this get taller? I can hardly get up...” She could swear it had only been a few months since she’d used it.

Where was her husband to help her? Right, he was in the market, with Sweetie Belle. Well, when he got back, she’d have him help. Of course, he’d have to shove her from behind, and his face had a habit of finding her rear whenever he helped, and she was already positioned for that. Sweetie Belle would need to play outside for a little while, in that case...

She sighed yet again. At least resting her forehooves on the lounge helped a little.