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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To be honest, Frisk found it difficult to enjoy work as much when she knew her boyfriend wasn't  feeling very well at home. Knowing him, he probably would get bored laying in bed all day, and might decide to get up while she was at work, knowing that she was unable to force him to stay underneath the covers to keep warm. Of course, while she fretted, the monster continued on to tell her not to worry about him, that he would be okay. He could look after himself when he wasn't feeling well, he had to before, so he was fine doing it again. 

She just didn't know if she was okay with it; somehow she had to respect his wishes and hope with all her heart and soul that he would be in good health without her. But... Her soul just wanted her to worry. She never really paid attention to her soul before, never even knew it was possible to feel it, or that souls even existed until she met Sans and he started explaining things to her. Now, ever since bonding her soul with Sans', all it wanted to do was think of Sans. The possibility of him suffering alone if she was unable to help him.

More days passed, and as she expected, Sans was getting out of bed even when he should have been resting. It had been a day longer than she had been unwell, and he was more grumpy than normal, weirdly distant, and to tell the truth it was a little difficult to deal with, just because she wasn't used to him acting this way towards her anymore. Sure, sometimes he could complain and growl now and then, but that hardly ever happened nowadays. Now, sometimes when she was at home, it was as if she was looking into a portal of the past where Sans was frustrated all of the time, and he didn't want to be bothered.

Maybe he was fed up of feeling unwell, and that was the conclusion she had drawn. She couldn't blame him for being mad about it, but at the same time she wished he would realise that she was right there for him to get him whatever he needed to help him feel a little better. Thankfully, to disrupt the misery from the skeleton, two visitors decided to show up, that being Muffet and Mettaton once again, just there to check up on them.

"Oh dear, what's got this skeleton in a sour mood?" Mettaton had wondered, as they were all in the living room and Sans was curled up in the corner of the couch, seeming to keep to himself, even during this conversation when it was about him. Papyrus spoke up, saying that he was feeling under the weather, so he was pretty miserable at the moment. He likely hadn't expected to feel bad for this long, and probably hoped it would pass pretty quickly, but it hadn't. 

"IT'S NICE TO SEE YOU THOUGH, METTATON, MUFFET. HOW ARE THINGS BACK AT HOME?" Papyrus decided to ask, mostly towards the robot, who smiled, as he mentioned he actually had some news to tell them. And that was what he had told the others back at the headquarters - for the time being Undyne had seemed to let go of looking for Sans, Frisk or Papyrus, and that she would focus on other things. Hearing this, both Frisk and Papyrus smiled widely, being delighted by such news. Sans wasn't paying attention to the conversation, so he didn't react at all.

"WOWIE! THAT'S AMAZING NEWS!" Papyrus whooped, glad that it seemed like it was more definite that they would remain more safe for the moment in time. That Undyne wouldn't be looking for them around every bend and every corner of the country or something like that. They were actually free to live their lives without having to stress so much about the fish tracking them down, and that was amazing to them.

"It really is. Now you dears can live for the time being as you please." Muffet told them.

"That's so great! Sans, did you hear what he said? We're going to be okay!" Frisk told her boyfriend, who snapped out of spacing out and he glanced towards her, once again not really even seeming to react. Well, he said a confused sort of "huh, what?", which caused Frisk to repeat herself just as excitedly, however his reaction to what she had said appeared flat, as if he suddenly wasn't interested.

"that's, uh, great." Sans said, going to get up from the couch. "i'm still not feelin' too good so, i'm gonna go up to bed." He finished, heading towards the stairs to head up to his and Frisk's bedroom so he could likely go to sleep. Frowning, the human girl stared after him in bewilderment, still confused as to why he was acting so strange. He had been so not like himself these past few days, and she couldn't stop wondering why. What was it that he was so affected by? Did he think that...

Her eyes widened slightly. Wait, did he think that maybe he wasn't feeling too good and that he was maybe going to follow the same fate as what had happened to his and Papyrus' mom? Such a possible thought horrified her to her very core, and she hoped to God that that wasn't what was happening right now, that he was actually extremely sick because of her and that he was dying. Dear God, there was no way that could be happening, absolutely no way.

"Um, you three can chat down here if you want, I am going to go check on Sans, just to make sure that he is okay." Frisk told the three others, who nodded - Papyrus being the most worried out of the trio, as he was Sans' little brother who had also been in confusion as to why Sans had been acting so strange and kept to himself lately, unlike his more happier self that was normally so overjoyed in the presence of Frisk. So, what was going on?

"Sans?" Frisk called, hearing the toilet in the bathroom flush, and afterwards, she saw the skeleton emerge from the bathroom, appearing to avoid her eye contact. "Sans, are you ignoring me for some reason? Are you mad at me because I made you unwell?" Frisk assumed, hoping that was the scenario that was going on here, just because it was so much better than the other one she had thought of inside her brain moments ago. 

"no, that's not it." Sans simply answered, causing Frisk to pull her face into a frown. He was being so vague and blunt that she was thinking she might not end up getting an answer as to why he had been acting so weird. She decided to push it, asking once again why he had been acting the way he had, and he continued: "it's not important."

"How is it not important? What's gotten into you, Sans? Because right now at the moment, I just feel like I'm talking to you from the past. Clearly it's important enough to be causing you to act like this, so what is it?" Frisk demanded, wanting to be a little tougher with him this time, hoping that perhaps with her harsher tone it would make him want to admit the truth to her. Standing by their bed, Sans stared down towards the covers, before slowly glancing towards Frisk.

"i think we should... i think... we should break up." Ouch. Okay. That was something that Frisk was not expecting. It stung, like fucking crazy, so much that she had to blink back tears at such a phrase. But... Why? Surely, he had to have a reason. All of this, it was so out of the blue, literally days ago, when she was unwell, he was his normal self, the one that had been so carefree about looking after her. Well, actually, he had been fussy of course - just his normal affectionate self. So... Why?

"What? Where is this coming from all of a sudden?" Frisk cried, harshly blinking back the tears. No, she wouldn't let herself break, and neither would she randomly let her and Sans end like this if he was really thinking of throwing this on her completely out of nowhere. She wouldn't take that at all! "What has gotten into you?! Lately you haven't even been acting like my boyfriend, but a totally different guy. Did the sickness get to your skull by any chance?!" Frisk snapped, seeing Sans avoid her eye again. She exhaled. "Sans... You can't mean that. Really. Please, just tell me, what is wrong?"

Hesitant, Sans was quiet, until his pupils slowly landed back on her ocean blue eyes, and in that moment when he had locked eyes with her, he suddenly broke, starting to cry as he crumbled to his knees. She didn't understand, not one bit, but that didn't matter. As he began to sob, she was at his side in a second, wanting to pull him in for a hug as he wailed. Okay, this was a step forward, maybe after he had calmed down, he would tell her what was up. "Shush... Shush, it's okay. I'm here for you, okay? Whatever is wrong..."

"i didn't mean that, i promise i didn't mean that." Sans sobbed into her shoulder, while she whispered beside his skull, saying that she knew he couldn't have meant it. "don't ever leave me, please." He pleaded, causing her to pull back from the hug a little, so she could look at his tearful face with a soft smile. Using her hand, she went to softly touch his cheekbone, gently wiping away a tear. 

"Never. I'd gladly remain with you forever, Sans. So please, tell me what is the matter... Whatever it is, I'll listen." She promised, seeing his pupils flit at her for a moment, before he stared down at his knees, seeming to try and calm himself down before he would start speaking again, otherwise he might not be able to get it out what he had to say so she wouldn't be so in the dark about his behaviour lately.

"frisk..." Sans' eyes landed back on her face again. Patiently waiting for him to speak again, she didn't say a word, just wanting to hear it, whatever it was, whatever he needed to get out, she was there to listen. He exhaled, going to wipe away his own tears, and with that done, he just had to say it. He knew he couldn't avoid it, even though he knew he had wanted to.

"i'm soul pregnant."