Well, that was what Marek was good for, really. It was why he never stopped saying the ridiculous things that came to his mind. They made people laugh, distracted people from the seriousness of life going on around them. It made him feel better to joke about his current condition, and clearly it was amusing Chas when he poked fun at her condition, too.
"I probably could've told you that," he said with a hint of a smile. "You stopped shagging me for him, remember? Long before you actually got any action, if I recall correctly. Thank Helga for that, too, or that spawn inside you might've been mine. Bet you'd get rid of it then." He had no idea what he would have wanted her to do in that situation, but that wasn't the point. He was grinning, fairly lighthearted, though not enough to make too light of the situation. "So you're keeping it, then? You're going to get round and bitchy, and then that kid is going to make this planet hell on earth. Yours and Barty's, fucking hell. Good thing the Dark Lord's out of the way, yeah? Just in time to make room for that little bugger you've got cooking away in there."