It can be done, if you keep the characters separate, and/or use sprites.
It's a matter of skillsets. Personally, I can't even hold a pen any more, but rendering takes time. Time to open files (for Daz, that can be up to 20-30 min right there), posing, swapping characters, and doing the renders. At best, you are looking at an hour or two per render, and you need an extra full render per character combination, per frame. And as I said before, this increases exponentially when you have multiple characters who can get pregnant and interact.
Even with StS, you are waiting until the end of each girl's arc., so there is less going on, and they can't do a lot of their normal things when they are pregnant, including them being in other stories. You notice that none of the students or teachers can get pregnant, only the ones who have their own, independent areas. They also speed up the act, and squeeze 9 months into 3 weeks (one of which they are not even showing).
Whereas with sprites, you make one normal set and one pregnant set, and just swap them out as needed. You still have to pose them, but there are no extra render/loading times, there is no massive file size bloat, nothing like that. It makes things far easy to deal with.
Don't get me wrong, if that is what your game is about, or you want to make it work, you can plan for it, and take precautions to make it easier, like not letting your characters interact with each other much, but most games don't really work that way.
And this is one of them, the girls help each other, there is the weekly event, and most of them would have their lives ruined by getting knocked up right now. That is why they did things this way here, they plan to add it later for the main girls (I assume), and in the mean time, they add other events, the dream, the pregnant friend, the girls in town, the magic cow girl, who you can repeatedly "knock up", and keep her in that state indefinitely, fucking her every day if you want (and the other you can 'milk' every morning), or waiting a week and doing it fresh every time. They added that new area, 3 girls, a story arc so you can grow to care about the main one some, and added that whole system, just for that kink to be fulfilled, and people still complain that it's not good enough.
...and people wonder why devs get burnt out.
