Sans is a MONSTER and not a HUMAN, so he doesn't follow HUMAN biology like a HUMAN skeleton. Same with Gaster and Papyrus. 

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"What?! No way!" Chara cried at the robot, who had decided to come back from Sans', Frisk's and Papyrus' house, actually letting them know exactly what Sans and Frisk had told him, Papyrus and Muffet. When the robot had still been at their home, the human girl had decided to come downstairs with her sudden nervous boyfriend, and the three others had just watched them in confusion - especially Papyrus - because he had been worried as to whatever was wrong with his older brother, and as to why he had been acting so strange. It took Sans awhile, but when he got out the same news he had told Frisk, Papyrus' eyes had almost popped out of his skull as he gasped in shock (and happiness) for his brother, while Muffet and Mettaton stared with interest.

"Of course when they go away that's when they have to get pregnant." Chara grumbled, seeming disappointed that she couldn't talk to Frisk about all of this, with her being too far away and meanwhile she was stuck here, in the Mafia headquarters actually finding herself missing her older (non-biological) sister. Everyone had been missing her, especially Toriel, who had enjoyed being a mother figure to her, she couldn't blame her mom for feeling sad about it, she was too. "Though admittedly I was expecting Sans to get her pregnant, not the other way around. Funny how things can go." Chara laughed, while the robot chuckled and nodded in agreement. 

"Though, it is a bit of a concern..." Toriel decided to speak up, who was currently at the headquarters herself, seeing as it got easily lonely nowadays on days where the kids weren't home, and Asgore had to work doing his job. To her surprise, her husband actually had been trying harder to try and get time outside of work, even if that time was still rare... But as much as it still frustrated her, she could see that he was trying. Upon her saying what she said, Chara and Mettaton glanced at her in confusion, wondering as to what in the world she meant.

"What do you mean?" Chara decided to ask.

"Well, Sans' soul isn't exactly in amazing condition." Toriel pointed out with a small thoughtful frown as she remembered the day all the way back when the idiot had sent himself into a coma, and she had to deal with giving him medical treatment that day. Frisk had been outside the room at the time, as she had asked Frisk to wait. But when she had to connect an IV drip that contained magic to his soul, she saw the mangled state that it was in, with all those cracks and the fact it didn't even glow as bright as it was supposed to. "It makes me wonder if he would even manage to carry it through... Let alone have the child." Toriel spoke, now getting worried glances from Chara at such a thing.

"But that's just me speculating!" Toriel decided to add quickly, noticing how troubled Chara appeared when she had said that, while Mettaton was now looking thoughtful at her words. She didn't want to scare everyone into thinking that maybe the child that they could possibly have might die, or he might die trying to have the child but... It was definitely risky in her opinion. Maybe it was best to let Sans and Frisk know that before they carried this too far. It's not that she didn't want them to have a family, (even if it was a shocker and she was somewhat frustrated they hadn't been more careful), she only wanted them to remain safe, knowing they would be heartbroken if such a tragedy happened.

Back at home, even though it had been days later since the news, it was very clear that the trio living in the house were still feeling weird and shocked about this whole ordeal. Truthfully, Papyrus' reaction had been a tiny bit amusing to watch to Frisk, just because he had been so shocked. "WHAT?!?!" He had yelled out in surprise, while leaping up from his seat almost ready to give Sans a hug of congratulations. Of course when Papyrus hugged him, Sans told him to be a bit careful as they had almost stumbled over from the amount of enthusiasm Papyrus had used in the hug. After that his little brother began quickly apologising, only for Sans to reassure him over and over that it was fine.

"It's still unbelievable this is happening..." Frisk murmured with a small smile, as her and the two other skeletons were sitting at the table, eating together after the youngest skeleton and the human girl had come back home from work. It was such a strange thing to get used to, the fact that she was actually fussing over her boyfriend because he was pregnant. You didn't say things like that every day! Sometimes it even made her giggle how weird it felt to say "he" instead of "she" when she was talking about the fact that Sans was carrying their child in his soul. Didn't that make him the mommy, and her the daddy? She almost snorted at the thought.

"I STILL HAVEN'T GOTTEN OVER THE SHOCK FROM WHEN YOU TOLD ME! SO, YOU'RE REALLY SURE YOU WANT TO GO THROUGH WITH THIS? NOT THAT I AM TELLING YOU TO BACK OUT OF COURSE, I THINK IT'S WONDERFUL!" Papyrus beamed, while Sans scratched the back of his skull with a weak and nervous smile as his two white pupils stared down at his half empty plate. Such a question had taken him awhile to decide on before he told Frisk, a couple of days exact, whether or not he had wanted to go through with this whole situation. It wasn't like he was any less fucking terrified than he had been before, but he knew that in this, he wasn't alone. 

"...yeah... i mean... i had thought about not goin' through with it. of not sayin' anythin' and just gettin' rid of it, but that wouldn't have been fair to frisk... would have likely been a shit thing to do too." Sans said with a sigh, while Frisk smiled back at him, touched that even while he had been worrisome, he had decided to also consider her feelings about this child. She knew she was afraid too, because this was all so new to her, this wasn't a normal human pregnancy where it would have maybe been more obvious what was right and what was wrong. This was different to her; new. But she knew there was no way she was as frightened as her skeletal boyfriend was, so she put him before herself in terms of terror. 

"Well, we're all here for you, Sans. Especially me." Frisk told him sweetly, which got a soft chuckle out of him. Right now, he was doing his best to remain as calm as he could, but if he was honest, there was so much he was still on edge about that him and Frisk needed to talk about. He hadn't really gotten the chance to during the day he told her what was up with him, because he was in too much of a state to really say what was on his mind. But he did have things on his mind.

"yeah... actually, i was wonderin' if we could go for a walk a little after we have eaten. just ya and me, frisk. there's some stuff i wanna talk about, that i didn't really get the chance to when i was... well, havin' a bit of a breakdown. heh." Sans said, which earned her raising her eyebrows in wonder in return, but then she nodded in agreement, liking the sound of a walk, and at the same time she wanted to know what ever was on his mind. She figured it was something to do with the pregnancy, as that was all that would be on their minds for awhile. Papyrus didn't at all mind that they wanted to walk alone, knowing they likely wanted to continue to talk about the fact that they were going to be parents. So, when the time came for them stepping out of the door to walk together, he gladly allowed  them to go.

Upon getting outside, Sans instantly linked his hand with Frisk's as they began to walk. "so..." Sans started, but didn't say anything for a couple of seconds, which caused Frisk to jokingly repeat what he said, only in a question, which made him roll his eyes as she flashed him a tiny smile. "this whole thing... firstly, i wanna thank ya for bein'... just really awesome about it, while i was havin' my whole melt down... truth be told i'm still in shock, but... i know we need to talk about it." Sans went on, causing Frisk to nod.

"Yeah... I wanted to be supportive, because you were very clearly upset. I felt a little bad for snapping at you afterwards when you said you wanted to break up, but then when I calmed I knew you couldn't have meant what you said." She replied, causing him to softly squeeze her hand. As he had said before, there was no way he could have possibly meant that in the state he had been in. No matter what, he would never, ever want to let go of Frisk. He would never want to lose her to someone else, he would never want to spend his days without her. 

"yeah... i'm still so sorry that i said that, i really, really didn't mean it. at all." He promised, clinging onto her hand as they continued to walk together, as he really wanted to make it clear that letting go of her could be the worst mistake he could ever make. Smiling, Frisk clearly got the message, and happily squeezed his hand in return, as she wasn't at all mad at him for what he had said. After everything, she understood that he had been so afraid, so she couldn't blame him for acting out. People did stupid things when they were scared. 

"anyway... what i wanna talk about is... well, firstly, i am now completely unable to use magic for the time that this is happenin'. reason for that is 'cause we're supposed to store as much magic as possible for the child... i know i couldn't really use magic anyway but... it means that i have less of a chance of bein' able to protect ya if anythin' bad were to happen in the time that this is all goin' on." Sans told her, causing her to shake her head with a sigh, while she continued grinning. That was just sooo Sans! Even now, when things were meant to be about him, he still worried about her. Now, it was her complete responsibility to look after this skeleton, no matter how much he wanted to try and be the protective one.

"That doesn't matter." She bluntly told him, causing the two of them to stop on the sidewalk. She smiled. "I know you always like to look after me, Sans... You like to try and be that tough skeleton who is invincible against everything... I won't deny that you're strong, you're stronger than you can ever give yourself credit for, even if you act like it's a weakness to break down and cry. But for once, please let me be strong for you. You've been through so much, you don't have to always keep taking everything on on your own. That's why we're a couple, that's why I'm here. Because I want to protect you. Not just you, but our child too." With a smile, she gave a small, brief giggle which caused Sans to look at her, completely surprised by her response. He thought she would have been worried, but she wasn't... She just countered what he said with her positivity, rather than agreeing with what he said... It was one of the many reasons why he loved her, so damn much. 

"i... but i..." He sighed. "...thank ya, frisk..." He replied. Somehow, he had to allow her to have the responsibility of looking out for the both of them. It wasn't that he didn't trust her, or that he thought she couldn't handle herself, it was just... It was other people he didn't trust. He didn't want one single hair on her head to come to harm because he wasn't able to protect her. 

But... Somehow he just... Had to give up that responsibility.