As much as I would like this also, it wouldn't make sense for a lot of these enemies. The zombies (base and boss) wouldn't effectively have something of the sort. While the female zombies has a pseudo one, that's just an infection tentacle for other females (as seen in the gos).
The dogs, the Demogorgon, rats, and whichever enemy will appear where it wouldn't really make sense to 'fast track' an "offspring".
The slime thing, absolutely since that's been done many times before with ease. Apart from that, this game is too... Quick in its story to have any actual resemblance of pregnancy. I would have loved it in Parasite in City, but you effectively survive throughout the one night.