A/N: Takes place in Harry's third year, but the book plot largely happens in the background.
Just his luck, Severus had to be in the wrong place at the wrong time with a transformed werewolf. Only his first month here and that stupid lycanthrope had forgotten to take his last dose of wolfsbane potion. When Severus tried to take it to him in time, he'd been too late, finding himself trapped with the wolf. It leapt on top of him, and Severus braced himself for a painful death, his punishment for attempting to keep the beast sane.
To his astonishment, however, instead of getting attacked and bitten, he was mounted and vigorously mated, ravaged by a wolf with no human sensibilities, only feeling the primal urge to breed. It must have smelled it on him, that he was fit to bear and a strong specimen, and therefore a good candidate to carry its issue. It went on for hours, the wolf repeatedly pounding his unwilling breeding mate.
By the end, the wolf was satisfied and asleep. Severus was panting on the floor, clothes torn, legs bloodied. It was all he could do to crawl out of the shack and stagger through the passage, back to the castle where he stumbled to his quarters and collapsed. Everything hurt. Certain parts were on fire. He was covered in sweat and fur and dried blood, with a generous amount of the wolf's seed inside him, and too exhausted to move.
And that's how the male bearer found himself pregnant by a werewolf.
He'd worried that he was going to end up with a belly full of cubs, so he was relieved to find out that he was carrying one human baby. That he could handle. Probably.
There was no point telling the headmaster about the incident - he'd made it clear years before that he didn't care if Severus got attacked by a werewolf, or by anyone. Nothing would be done to punish the offender. Besides, Severus didn't want anyone finding out the whole story. It was enough for him to have to come to terms with, both mentally and physically.
When his persistent morning sickness got Pomfrey's attention, she insisted on checking him over, and discovered he was in the family way. She was naturally curious as to how he'd gotten pregnant and who the other parent was, but Severus insisted on keeping it a secret.
He had several months to decide what he was going to do, whether to keep the child or give it up for adoption. Sometimes he thought it would be best to let someone else raise it - he was hardly parent material. Who would want him for a father? Other times, he wondered if it would be right handing off a child with werewolf genes to someone else who may not know what they were getting into. It wasn't a guarantee that the child would inherit lycanthropy, but the possibility was there. Severus was no expert on childcare, but he did know a thing or two about werewolves, at least, and he was definitely qualified to brew wolfsbane, while many weren't, and the potion and ingredients weren't that easy to come by. He debated back and forth for a long time, deciding one thing and then the other, over and over.
Meanwhile, Lupin remained blissfully unaware that he had inadvertently sired a child, much less that Severus was the one carrying it. Keeping such a big secret right under his cold wet nose gave Severus some satisfaction, at least. He liked any opportunity to get one up on Lupin, and he knew that the man believed he would never be a father. And he wouldn't. He would never know that whether he got the last word in or not, Severus had the last laugh as he swanned off carrying what Lupin wanted and couldn't have. In fact, knowing Lupin might want the child nearly swayed Severus into keeping it, if only to ensure they were kept out of his bumbling paws. Besides, he was busy every month turning into a raging, hormonal wolf. Surely a worse choice than Severus for raising a child.
His first trimester had been awful. Morning sickness sent him scurrying to the nearest loo every day, making Pomfrey worried that he wasn't keeping anything down. Severus was too sick and tired to work sometimes, and in no condition to care. There were many days he simply lay in bed miserable, nibbling on crackers or sipping broth and tea and cursing out werewolves everywhere. The glares he sent Lupin's way when he did see him were more venomous than ever, and people wondered how Lupin didn't drop dead on the spot. He could only guess that he had done or said something to make the potion master hate him more than usual, but Severus wasn't telling him anything. He managed his nausea and fatigue as best he could, alone, soothing himself by dreaming up all the different ways one could murder a werewolf, or creative methods of neutering.
Another consequence of his condition was that he could no longer brew wolfsbane - it contained aconite, and he couldn't handle toxins while pregnant. Not to mention, at least not to Pomfrey, that Severus imagined it could be highly dangerous for him to come into contact with wolfsbane while carrying a werewolf's offspring. There was no way of knowing yet if the child had inherited their sire's furry problem, but it was a risk to consider nonetheless. Thankfully he didn't have to mention this for Pomfrey to agree it was safer for her to take over brewing the wolfsbane, though he passed along some very detailed instructions on brewing it correctly. They didn't need an out of control werewolf again.
To Severus' great relief, the debilitating nausea ebbed away as his second trimester arrived, and he could focus on his cravings for meat and his expanding stomach, which came with new issues. Before long he had a bulging middle that pushed against his clothes. He worried about how he was going to hide this. Pomfrey had told Dumbledore about their potion master's condition, who thankfully hadn't spilled the news to the entire school, and Severus hoped no one else would find out. He had an image to maintain, which had nothing to do with being at all paternal. He found a glamor that was safe to use, and wore it every day while in public. No one realized that their slender, cranky potions master was steadily growing thicker with child, or that he was wearing bigger robes. Pomfrey had half jokingly offered to find him some tasteful maternity wear, and Severus had not jokingly offered to hex her.
He'd never had much sympathy for Lupin's furry problem, yet lately during full moons Severus was getting stomach aches, a churning that only happened at night, and didn't let up until morning. It couldn't possibly be sympathy pains, and Pomfrey only said some indigestion was normal during pregnancy. But he didn't get them any other time except during the night of the full moon, a fact he hid from her. He hated to think what it meant.