Turns out being pregnant with angelic eggs was a lot like being pregnant with a human baby. Though gestation (or in this case, "incubation") was slightly longer. Cas had explained that this was because their offspring needed a vessel to contain them just as Castiel did and were developing their own bodies in addition to their angelic grace, using DNA from both Dean's and Castiel's semen.

"But you're an angel," Dean had pointed out. "Your DNA is the grace, isn't it? The jizz you shot up my ass will have Jimmy's DNA."
"This is my body now," Cas said, self-consciously adjusting his coat. "Jimmy has been gone since Lucifer tore us apart in that cemetery. Yes, the DNA in what I ejaculated into you that will help determine what our offsprings' vessels will look like will mean they will look like you and Jimmy Novak. But everything that is me is in the grace within the eggs. They are just as much a part of me as they are you, Dean."

Dean decided not to question the eggs' parentage again.

And, keeping with the theme of "a lot like a human pregnancy," the first thing to start happening because of the eggs was morning sickness.

"Son of a bitch," Dean groaned between retching into the toilet somewhere into the latter weeks of the first month. "I'm not even technically pregnant, am I?"
"Not in the traditional sense," Cas agreed where he knelt beside Dean, reaching out to rub soothing circles into the hunter's back. "But your body still recognizes the eggs within you as foreign intrusions and is trying to get used to them, as it were."

Dean remained quiet for a moment, thoughtful, a hand moving down to splay over his abdomen. "What if my body decides it doesn't like them?" he finally asked, barely more than a whisper, before meeting his angel's eyes. "Cas... is it possible that I could lose our kids?"

Cas's eyes flicked away from Dean's for a moment, clearly not wanting to think about an outcome like that, before he nodded. "Yes. It is a possibility as with any pregnancy," he said, meeting Dean's eyes again. "But that possibility is no greater than the possibility that you carry to term. Don't dwell on what may happen just because you don't want it to."

Dean nodded slowly in understanding then grimaced as he caught sight of the contents of the toilet bowl. Nearly gagging again out of disgust, he reached up to flush his breakfast away then slumped against the nearby wall, scrubbing a hand over his face.
"Hey. Cas?" he said after a moment. "What do the kids-"

"Fledglings."

"Whatever," Dean sighed. "What do they look like? Inside the eggs, I mean. You said it takes so long because they're developing bodies, too. Are the eggs going to… to have…" He held his hands up approximately the width apart of a newborn. "You know."

Cas tilted his head and squinted at the space between Dean's hands, clearly confused by the question. "Do the eggs contain infants? Yes," he said when Dean nodded. "As I said, they need their own vessels just as I do. But theirs will be completely their own."

"So when I lay them… and they hatch-?"

"A full term infant will be inside," Cas concluded. "When you've laid them, I will use my grace to begin breaking the shell to free them."

Dean pressed a hand to his mouth as his stomach threatened to rebel again. He wasn't quite sure how he felt about the idea of pulling an infant from an egg. Then again, the only reason this bothered him was probably because it was different than what he'd known pregnancy to be for humans. And while Castiel looked human, he wasn't. And neither were their children. So just because this process was something new, it wasn't necessarily wrong.

"Okay," he said on a sigh, dropping his palm to his denim clad thigh with a slap. "I can do this. Will, uh… Will being in the eggs outside of my body hurt them? I mean, they're not gonna suffocate in their, are they?"

"They will not begin breathing on their own until they've been extracted from the eggs," Cas explained. "However, once they are outside of your body, it is unwise to leave them in their eggs for too long. Without your body, the egg will begin to decay and the infant inside will die along with it."

Dean swallowed around a wave of nausea that had nothing to do with morning sickness. "Okay, so, get them out asap; got it."

"We can do this," Cas assured, gripping Dean's arms to help the hunter rise as he started to push himself up, rising with him. "I know it will be hard, but you are one of the strongest humans I know, Dean."

Dean made a vague grumbling sound that probably could have been a "thank you." Then, seeing the slightly crestfallen expression on his angel's face, he said, "Cas… I know I suck at moments but... thanks. I know I'll probably resist you on things and snap at you along the way but just know... I do appreciate everything you do and will try to do for me," he said quietly then stepped closer to his angel and placed a chaste kiss to Castiel's jaw, unwilling to kiss his angel properly until he'd brushed his teeth. "And… I love you."

"I love you, too," Cas said, unable to resist a small smile.

"Hm. Now let me brush my teeth."

"Can you tell how many are in there?" Dean asked late one night as the two lie side-by-side in Dean's bed facing one another. "'Cause we're probably going to need to name them, right? We should probably know how many names we should think of."

Cas nodded. "I could if I actively tried looking. Lying like this, I can only vaguely sense their energies within you but it is too muddled together to discern between each offspring."

"Can you look?" Dean asked, rolling onto his back to expose his abdomen more.

Cas nodded again and shifted up onto one elbow, reaching his other hand over to press against the still flat plane of Dean's abdomen and closed his eyes as he focused. "I don't know for sure how many I deposited into you but I can sense three distinct grace-soul signatures," he said as he retracted his hand and lied down beside Dean again.

"Three, huh?" Dean said thoughtfully. "We can do three, right? I mean, it's not as common as singles or twins, maybe, but we can handle that, you, me, and Sam?"

Castiel nodded, smiling reassuringly and reaching over to clasp one of Dean's hands in a strong grip. "We can. It will likely be challenging but so I'm given to understand that's a prominent aspect of parenthood in general."

Dean huffed an amused laugh and squeezed Castiel's hand. wiggling slightly closer to the angel with a sleepy sigh.

"Hey, Cas?" he said after a few more moments of silence.

"Yes, Dean?"

"We should give them a nursery. Maybe redo your old room since it's between ours and Sammy's," Dean suggested.

"That is a nice idea," Cas agreed.

Dean hummed faintly, slowly losing the battle with sleep. As he drifted off, one hand found its way to rest atop his abdomen.