It was a humid spring day in Canterlot, and class at Princess Celestia's School for Gifted Unicorns was wrapping up before the weekend.

"Remember, class, to read the second half of Chapter 8 before Monday, and be sure to take comprehensive notes, because there will be a quiz."

The room was filled with the hum of magic and the ruffling of papers as the students packed up their bags to leave.

The teacher cleared his throat. "You haven't been dismissed yet. There's still one last thing to cover. The winner of last month's essay contest has been chosen." He used his magic to lift an envelope from his desk and tear it open. "Twilight Sparkle, please report to Headmistress Celestia's office to claim your prize. Class is dismissed."

The students resumed gathering their belongings and streamed out the door, Twilight taking a detour to Celestia's office. When she arrived, the door was already open, and Celestia was sitting at her desk, filling out paperwork. Twilight cleared her throat, and Celestia looked up.

"Ah, Twilight, what a pleasant surprise! To what do I owe the pleasure of your visit?" She smiled and gestured for Twilight to come in.

Twilight walked into Celestia's office. "I won the essay contest. You mean they didn't tell you?"

Celestia shook her head and closed the door with her magic. "No, they didn't. After you won the last few contests in a row, we decided it would be best to keep me out of the loop until the winner is announced, to make it clear I'm not playing favorites."

Twilight blushed. "Well, equine development is pretty well understood. All I had to do was do some research, citing my sources of course, and the paper wrote itself."

Celestia nodded. "The biggest factor is effort. The median number of sources cited was three and a half, compared to your eight. You can be a little too modest sometimes."

"I just feel like the other students can't compete." Twilight sighed. "Anyway, based on the other times I've won, I'm guessing you have an experiment for me to help with?"

Celestia smiled proudly. "Excellent induction, Twilight! Indeed I do." She cleared her throat. "While equine development is indeed very well understood, there is unfortunately a rather large gap in our knowledge which I hope to fill. Can you guess what that is?"

Twilight shook her head.

"I'll give you a hint. What's the earliest thing you can remember?"

Twilight thought for a moment. "In preschool, somepony came in and did a magic show for us. I was really impressed."

"And now, you're studying magic yourself. I'm glad I was able to inspire you." She cleared her throat. "Nothing before that?"

Twilight nodded. "Even that is kind of fuzzy."

Celestia smiled. "Well, that's the gap I hope to fill: we lack a first-person description of the stages of development we cannot remember."

Twilight nodded. "So how do you plan on filling in that gap? It's not like we can just ask a foal."

Celestia chuckled. "It's funny you should say that. She opened one of her desk drawers with her magic and retrieved a small glass bottle of iridescent potion, which she set on top of the desk.

"What's that?" Twilight asked.

Celestia uncorked the flask. "With the help of a visiting zebra, I've prepared a potion to undo the aging process. We couldn't make enough to make me as young as necessary, but you're young enough that this much will be plenty." She shook the bottle.

Twilight picked it up with her magic. "And how young will I get, and how long until I can return to normal?"

"I'd like for you to at least document your first year, because a lot happens then. If you're up for it after then, you can take it further, up to the point when we have first-person accounts already."

Twilight nodded and absentmindedly swirled the flask with her magic, watching it glow. "I'll keep my memories and everything?"

Celestia nodded. "Yes, of course. The experiment would be useless if you didn't, and I don't want to set back your schooling."

Twilight brought the mouth of the bottle to her mouth, took a sip, and winced. "It's really sour."

"Yes, that's from all the magic it contains. Drink it quickly and the aftertaste shouldn't longer for too long."

Twilight tilted the bottle up and chugged it as quickly as she could, burping when she was done. "It tingles."

Celestia nodded. "That will get intense, just to warn you."

Indeed, the tingling soon turned to pins and needles, soon joined by the sensation of falling. She swayed on her hooves and steadied herself against the desk.

"You'll be alright; the discomfort will pass momentarily. Just hang in there."

The sensation gradually subsided, and Twilight took a few deep breaths to calm her nerves. When she looked to Celestia, she had to strain her neck more than usual. Looking down, she could see that she was indeed shrinking. "It really works!" Twilight exclaimed with the adorable voice of a little filly.

Celestia nodded and lit her horn and touched it to Twilight's. "So we can still communicate once you're a foal, I've established a telepathic link between us. Try it."

Twilight closed her eyes to try to concentrate and noticed that her eyelids were glowing slightly in fact, her whole body was. «Can you hear me, Celestia?»

Celestia nodded. «Loud and clear.» She lifted the young foal with her magic, setting Twilight down on her desk. «You should be done in a minute or so. You're doing great.»

Twilight smiled, her body now that of a foal just a few months older than a newborn. «Will I still be able to continue my other studies during the experiment?»

Celestia chuckled. «It was actually intended to be a vacation for you, but I'm sure I can find the time to teach you.»

«Thank you *so* much!» She reached out to hug Celestia, but all she could manage was a nuzzle.

Celestia watched as Twilight shrunk down to the size of a newborn foal. «You're welcome.»

Twilight looked at her tiny hooves. «How long until I'm done shrinking?»

Celestia blinked. «What? You should be stopping right about now.» When Twilight continued to shrink, Celestia's eyes went wide. «Uh oh…»

«‹Uh oh›? What do you mean, ‹Uh oh›?»

«You're still getting younger.»

«What? Younger than a newborn?»

Celestia nodded. «It looks like you're an unborn foal, about a month before birth. This wasn't supposed to happen; I told him… Oh.»

«‹Oh›?»

«Zebras count age starting at conception, not birth. It seems there was a terrible misunderstanding.»

«You mean I'm gonna to keep shrinking until I'm a zygote? Celestia, we need to stop this potion; an embryo can't survive on its own!»

Celestia sighed. «Your body has already absorbed the potion; it has to run its course before we can do anything.»

Twilight felt her breaths getting more difficult as her lungs regressed. «I don't want to die…»

«I won't let you die; I promise you I'll think of something.» She bit her lip and tried to think through her nerves. «Twilight, I need to put you into my womb.»

Twilight was too weak by this point to protest, but she was still conscious enough to feel Celestia pick her up with her magic and see Celestia's swollen marehood approach her. Then, darkness.

Celestia carefully pushed Twilight into her vagina, already lubricated from heat. She knew had to hurry and get Twilight past her cervix before she was small and fragile enough to be crushed by her powerful muscles.

Twilight was just vaguely aware of her surroundings at this point, being completely cut off from the air outside. She could feel Celestia's contractions pulling her in, like a snake swallowing its prey.

Celestia panted in exertion, pulling as quickly as she could without injuring Twilight. She hoped this would work, that she would be able to save Twilight. She didn't want to admit it, but she was just making things up as she went along.

Twilight was drifting in and out of consciousness. Her mentor's heartbeat pounded in her ears, and the heat and moisture were nearly unbearable. After what seemed like an eternity, her forehooves bumped up against a wall of firm flesh, or rather a ring, a ring that slowly spread open. She saw a soft white glow as her head was engulfed by Celestia's womb, and then she blacked out.

Celestia sensed Twilight's loss of consciousness telepathically, but she could also sense that Twilight was still alive. Less than a minute later, she felt Twilight attach to her womb and breathed a sigh of relief.

A few minutes later, Twilight came to again and looked around. She was connected to Celestia with a pulsing umbilical cord, and the walls glowed with a very soft white light. Once she'd regained her strength, or what little of it was left, she kicked. «I'm okay, but I still feel tingly. Will I really shrink all the way to a single cell?»

«Unfortunately, yes. That will take five to ten more minutes, and then you'll start to age normally again.»

«And then you can get me back to normal, right? I mean, or back to a newborn so we can do the experiment?»

Celestia sighed. «No, I can't get a potion to you properly while you're gestating. I'm afraid you'll have to get there the long way.»

Twilight kicked in protest. «What‽ The full eleven months? That's not fair! I don't want to be stuck in your uterus for almost a year!»

To Celestia, Twilight's kicks felt like nothing more than a light fluttering inside her. «I'm sorry, Twilight, but you're attached to me now. You body needs mine to stay alive. On the bright side, you'll be able to record even more data for our experiment.» She tenderly rubbed her pregnant belly and felt it continue to shrink under her hooves, until the bump disappeared completely. It was going to be an interesting year.