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This artwork was commissioned by Kassaz, who wrote the following short story about it, part of this anthology:
https://www.fimfiction.net/story/568567/2/sweaty-short-stories-with-pregnant-pony-workouts/wingin-it

Wingin’ It

Rainbow Dash soared through the open skies, Ponyville below her. Perhaps “soared” were an exaggeration; “hovered” was better, “fluttered” perhaps. These thoughts raced through Rainbow Dash’s mind as she reflected on how her body couldn’t race at all in its current state, but she was going to correct that. She had gained just a little weight lately, and could feel it tossing and thrashing around in her belly. Nopony ever told her life would be easy as a mare, but she was determined to carry her burden with speed and style. Each tribe had some difference, even in the womb: Earth pony foals were really strong, unicorn foals could messily cast magic, and pegasus foals had two extra limbs to squirm with. She winced.

Finally, Ponyville’s gym was beneath her, and she slowly, steadily, embarrassingly lowered herself to the ground; it wasn’t with a swoop and a smooth slide to a halt as she could once do with ease, but instead a straight drop down as she slowed her flapping wings. She felt like she were carrying cargo, and didn’t dare turn her head to glance at herself and confirm that mental image.

Her hooves touched the ground, and she had to catch her breath. A glance behind herself without thinking made her take in the sight of her body once again. She wasn’t fat, but her cutie-mark had a slight curve as it wrapped around her rear that it didn’t have before. She already loved the shape that was inside her, but already hated the shape that she was in, and knew pregnancy was no excuse to let herself go like some mares did. Her husband had teased her about turning her into his fat broodmare, and it had been really hot at the time when they were making the foal, but she wasn’t going to let that happen! She’d keep up her exercise regimen as best she could and be back racing while the foal was still nursing.

Rainbow sauntered into the gym. It had been all too easy to avoid her usual visits, and she could feel the results jiggling on her butt with each step. Rainbow Dash was for once glad not everypony was looking at her. Now the question was what to do. She could take walks more often to keep her legs in good working order, and really shouldn’t do anything too stressful—pulling something and becoming unable to exercise at all would just make it worse, on top of possibly hurting her little passenger. Like her foal, she had two extra limbs on the average pony, and decided wing training would be best. They were the strongest part of her, and would need to get stronger if she didn’t want to risk grounding her home for a few months; they were also easy to exercise independently and, in the worst case, straining them wouldn’t stop her from burning calories some other way.

Usually, she’d have tied a weight to her body, or held one in her forehooves while she hovered, but could tell that clearly wasn’t going to work. It would probably be easiest to hold the weights directly against her wings. She put that thought through her mind’s eye, and winced at all of the ways it could go wrong. She’d need a spotter—she looked around for some horns—a unicorn spotter.

Fortunately, she saw Twinkle Shine, a fellow mare, and avoided the awkwardness of asking a stallion to spot her in her heavily-pregnant condition. She walked over to her while ignoring the foal movement that gave her rear steps a longer duration and wider swing for just a moment, and hoped Twinkle wouldn’t tease her. “Hey, Twinkles, mind if I ask you to spot me once you finish up here?”

Twinkle was stomping her hooves in place with weighted bracelets on each leg, and holding a weight in the air with her magic. She turned to Rainbow with a smile, which faded as she drank in the sight of her friend. “Um, sure, I guess?” Twinkle Shine lowered the weight to the ground and started to remove the bracelets via her magic. “Are you sure you should be exercising, like that?”

Rainbow huffed just a little, before she could stop herself. “I know what I’m doing. Pegasi have a proud tradition of athleticism.” Rainbow thought about adding to her argument, but decided against it; she was proud of herself for using a big word.

Twinkle Shine had another question as she followed Rainbow Dash to the weight rack. “Uh, what exactly are you planning to do?”

Rainbow grabbed a likely-clean sweat rag as they walked across the gym. She set her forehooves on an elevated platform before turning to answer her. “I’m just doing some wing endurance training.” She pointed to the nearby weights. “Just hover four of those weights over, two on each wing, and catch them if I start to drop ’em, alright?”

“If you’re certain.” Twinkle knew how Rainbow Dash acted when it came to feats of strength, but hesitated, prompting Rainbow to reassure her.

“I can totally handle it. I wouldn’t even need a spotter if it wasn’t for this little squirt. My mom told me she exercised when she was carrying me.” Twinkle thought that last little bit explained a lot, but decided to not argue with her. She started to hover over four weights; Rainbow extended her wings, widened her stance, and braced herself. Rainbow grunted as the magic dissipated and left only her wings holding them up. Twinkle kept her magic ready to call out and catch the weights underneath each wing if needed.

A few minutes later, Rainbow spoke. “Alright, could you grab ’em again real quick?” Once Rainbow felt relief and saw the weights lowered to the platform before her, she let her wings droop against her belly. Her belly relaxed in turn, allowing movement from her upset passenger to be seen. “Wew, thanks Twinkles. Just a few more sets and I’ll let you go.”

Rainbow Dash looked like she’d flown through a rain cloud. Sweat coated her body, and some had collected into a puddle beneath her. She was feeling the burn, and had just about had enough. Her foal had been exercising in her gut, kicking every time she relaxed the strong muscles lining her womb, and the strongest kicking yet was happening now. Rainbow felt her foal kicking so hard her muscles couldn’t hold it back. “Oof! Wing kicks… really hurt.” She heard Twinkle Shine gag.

Rainbow yelped when she felt her wings start to cramp and give out, but that was the entire reason Twinkle was there to help, and without asking she grabbed the weights in her magical hold and set them back on the rack after drying them off. Afterwards, she looked at Rainbow’s belly, still with a painful protrusion poking out from it. “Um, are you okay?”

“Y-Yeah.” Rainbow rubbed a hoof over her foal’s wing until it subsided. She exhaled. “My foal’s wings are so strong, so awesome!”

Twinkle Shine was never going to have a pegasus’ foals.