It's the Day's Day of Days! Because she's got spiders inside her head.
In the past, pregnancies such as this were bad enough in their blatant design to let Muir indulge both his emotional need to make Captain Mitty a paragon of virility and his barely-hidden breastfeeding fetish. But now that he's fucked up the cast's ages in the manner he has, it becomes a tangled mess that demonstrates what an absolute shambles he's rendered his fictional universe. Remember, aside from aging up the daughters to the point where the year they would have been born is before not only their original in-universe births but before their parents' original in-universe marriage, he's also retconned Sam's age outright once, and I'd argue he's been messing with it subtly since then.
All of which this latest plot puts on display. Not that he cares. I await in a few years when Sam is pregnant again, along with one of her daughters, and one of her granddaughters, who was born three years ago in strip, and is now suddenly eighteen.