Kassaz commissioned this artwork and wrote this short story to go along with it, also available in this anthology:
https://www.fimfiction.net/story/569242/3/tiny-stories-with-huge-pregnant-ponies/fertile-flock

Fertile Flock

A group of pegasi is called a flock. It was said that the militaristic pegasi of old had always been predisposed to singleton pregnancies, with the occasional mare finding herself predisposed instead to swelling with young much like the other tribes. Such mares were taken from the precipice of battle and sequestered in their floating cities where they would be more useful for replenishing their numbers. Many times in her life, Rainbow Dash had teased her friend Fluttershy about this, telling Fluttershy that she were obviously a broodmare. Fluttershy had long ago found it easiest to agree with her friend and talk about how pleasant it would be to have many foals. Rainbow Dash often reflected on this when she woke up in the mornings, pinned to the bed by a brood that singled her out as such a mare. Rainbow had previously believed her place in old pegasi society would’ve been soldier, perhaps even commander, and had almost hated to learn that she would almost certainly have been a broodmare who spent her days eating for foals, nursing foals, caring for foals, and making more foals with either the interested pegasi stallions or the captured stallions of the other tribes, interested or not.

Rainbow Dash thought about this for the thousandth time as she hovered above Ponyville moving at a speed even Fluttershy could best, corralling clouds with a paper bag of food held between her teeth. There was nopony around to watch her flying like a hummingbird, and that she preferred. As she slowly fused the clouds into a cushion, she continued to grumble to herself about joining the Wonderbolts, marrying a fellow Wonderbolt, getting knocked up with a flock of future Wonderbolts, and getting put on leave from the Wonderbolts. She had known a pregnancy would slow her down, but she’d held on to the idea that she could mostly ignore it, for the first few months. She had even asked Rarity to expand one of her Wonderbolts uniforms for nothing; there was no racing as big as she was, and certainly no ignoring her massive womb. Rainbow was thankful for her wings; she was slower than usual, sure, but she didn’t have to waddle around everywhere like grounded mares did.

Rainbow just didn’t understand it. She had no siblings, and so couldn’t see how she’d inherited her broodmare nature from either parent. She ignored how she’d teased Fluttershy despite her also having no siblings. Twilight had tried to give Rainbow a lesson on the topic, but Rainbow had mostly ignored the unicorn’s presentation. All she could remember were the paintings of broodmares over history, and how huge they were. Seeing a fancy historical painting of Princess Celestia huge with foals had done little to help her mood at the time either.

She didn’t want to risk tossing the bag of food onto the cloud only for it to fall through—she wouldn’t be able to beat it to the ground if it did—so she hovered over the cloud to land on it before letting her hindlegs slide and her butt slam down. The cloud held. She let the bag fall onto her bump now serving as her table and stuck her muzzle in to pull out a sandwich, which was gone in no time at all; she pulled out another, followed by the last. When she was done, she knocked the bag off her body, onto the cloud. She was still hungry, and stuck her hoof in her belly accusingly. “You guys are making me eat so much.” Her stomach spoke for them, with a long growl. “All of you had better grow up to be fast racers.”

One or several of them shifted inside her, causing some deeply uncomfortable pressure deep in the part of her body that didn’t belong to them. Rainbow spread her wings and lifted herself from the cloud until the feeling went away, and stopped flapping them to slam her butt down again. Time slowed for her as she felt her butt breach the bottom of the cloud, and she felt her belly leave the cloud last. It strained her wings, but she easily glided to another cloud formation underneath her but still very high in the sky. This time, she slammed her body down and could feel the cool breeze blowing against the bottom of her bump. She sighed and closed her eyes. It wasn’t resting on her stomach, but it was closer than mares of the other tribes could get. Rainbow had learned of pegasus broodmares who later returned to the front of battle rather than live lives of leisure. Rainbow Dash would be like those mares.