When it comes to Steven's birthday, Spinel is always happy to celebrate the fact that he's become one year older than the previous year. At first, today on Steven's 14th year of being alive, everything seemed to be going just fine.

That was until the moment when they surprise Steven with a cake, they (mostly she and Connie) were shocked to see that Steven grew a bit in height. Of course she asked Steven what he was doing, but he simply told her he just grew. Spinel didn't like how this birthday was turning out.

Especially the moment came when Steven and Connie danced, she got this… incredibly awkward feeling in her gem when she saw Connie lean into her boy, commenting to Pearl that she wanted to 'slightly move them apart'. But she really didn't want to ruin Steven's special day, so she followed Amethyst and Greg to the back of the barn.

"And why should I hold the pinata?" Spinel questioned.

"Just imagine!" Amethyst laughed, holding her hands up, "Steven's gonna rage when you keep moving it away from him!"

Spinel sighed, 'It's fine to challenge Steven, but I'm not going to be the one to prank him, he's my-"

She stopped, along with Amethyst and Greg as they saw Steven shrink himself to his normal size, only noticing them a moment after.

Spinel dropped the pinata, "Steven? What is this?"

Steven quickly shape-shifted himself back to being tall again, "N-Nothing! All is fine!"

"No, it's not," Spinel approached him, "Have you been doing this all day?"

Amethyst jumped in, "Dude! You can't keep doing that forever! Not even Spinel does that!"

"Why can't I?!"

"Your body can't withstand to be shape-shifted for a long period of time. Not even taking a minute break is gonna help."

Steven backed away from the pink Gem when she got close, trying to look away from the concern in her eyes, "But Spinel! Connie likes me this way! And how would you know? I'm half-human, maybe I work differently than you!"

Spinel couldn't speak.

Greg spoke up, "Son… this really isn't like you." Steven didn't reply, and tried to walk away.

"Steven, no," Spinel gently grabbed his arm, "I won't let you do this to yourself. I'm going to tell Connie-"

"NO!" Steven moved his arm away, "It'll be fine! I can do this!"

The boy ran off, prompting Spinel to call his name, to no avail. She could only sigh solemnly.

Spinel was walking back towards the barn, only for her to hear a familiar noise, her pigtails perking up.

When she ran closer, she found the Greg, Connie, and the Crystal Gems all huddled around the back of the van.

"Hey, what's-" Spinel was about to ask, until she saw Garnet holding… a tiny version of Steven, crying in his oversized shirt.

Just from the sight alone, Spinel's maternal instincts kicked into overdrive and swiftly stretched her limbs over to curl them around the baby, snatching it away from Garnet and pulled the baby to her chest, taking a few steps back from the shocked group.

"W-Whoa!" Connie gasped.

Spinel ignored her, giving all her attention to the baby, "Stevie? What happened?"

Steven continued to cry until he stared at Spinel. Smiling lovingly, Spinel cradled him in her arm, gently and playfully poking his chest. Steven began to giggle and grabbed her finger, trying to suckle on it.

"Wow, Spinel, you made him stop-"

Connie stepped closer to look, but Spinel noticed and protectively held Steven away.

"Um… Did I say something wrong?" Connie turned to Greg.

Greg chuckled, "Don't worry, Spinel's always been protective of Steven, especially when he was a baby. But hey, she got him to stop crying."

"Wow," Connie continued to stare at the baby held in Spinel's arms, "You really helped Greg take care of Steven?"

Spinel nodded, "Of course, I would even often skip missions just so I can see his adorable little face." She wiggling her finger to his face, making him laugh and coo.

Connie nodded, "No longer he's so attached to you."

With everyone else was taking care of the rest of the party, Spinel and Connie were in the barn, the former laying on a make-shift nest with hay while the latter was laying her chin on her crossed arms.

"... When do you think he'll change back?"

"I don't know. … Do you want to go home? I'm sure Greg won't mind driving you back."

Connie shook her head, "No thanks, Miss Spinel. I wanna stay."

"Hm, if that's what you want," replied Spinel, Steven pulling himself upwards and babbled at the girl.

Connie held out finger out to him, "Don't worry, Steven. I still wanna hang out with you, no matter what age you are. Your dad's got his car wash, the Gems have to do Gem stuff, but hey, you've got Spinel, and you've got me too. I can come see you when I'm done training with Pearl, doesn't that sound fun?"

Steven giggled and grabbed her finger. Spinel appeared satisfied, even wrapping an arm around Connie and pulling her in for a side hug.

"Welcome to the Gem family, Connie."

So I guess this not only about Steven becoming a baby and Spinel reacting to it, but it could interpreted as Spinel welcoming Connie wholeheartedly :)

... Which means she's gonna get protective of her too...