The problem I see is they do not set a proper Goal or Victory Condition for the Gameplay which makes the Simulation and Game Mechanics pretty aimless.
They usually don't have an Opponent or Challenge they have to overcome.
The problem with pregnancy is it's "realistically" pretty long and the only difference would be a change of state and unlocking pregnant sex scenes, that can get old pretty fast. Even after birth "realistically" a baby doesn't do much. This is why you see pregnancy in Ending/Game Over scenes where they make much more sense.
You could also have instant or accelerated pregnancy but again that doesn't do much other than increasing your pregnant counter.
Where things makes more sense is with Monster Girls and Monster Breeding where they can because Summons and Units that have gameplay effects, where they can go deep into RPG style genetics, stats, traits and abilities.
I have project I am working on I would like to hear your thoughts on what could be done more with Pregnancy Mechanics. I find how Pregnancy is implemented in a game like Corruption of Champions pretty disappointing.
There are a lot of ways you can add to pregnancy mechanics. If there is a trait/kink/fetish system, you could play around with that. Throughout the course of the pregnancy you could play with the MC's feelings on things do they hate being pregnant, maybe they enjoy some things about it not others, maybe they love it, this could then change potential flavour text in future encounters.
There could be some potential downsides to being on some form of Birth Control, maybe some slight debuffs here or there that could be balanced potentially around similar types of debuffs happening throughout some ones cycle, but not all the time like it would be on Birth Control.
Potentially children could add some financial strain on the player or maybe it could be a way to gain money through baby trapping well off NPC's or even be a way to have a different type of playthrough if you want the MC to be devious, for example baby trapping a random strong NPC so they can beat the "bosses" you should have fought but now the baby's father doesn't want to risk you dying to those "bosses" because then they have to raise the kid by themselves.
Maybe certain NPC's have variations on pregnancy kinks that could unlock different scenes only while xyz variables are met, so for instance you have to interact while being off birth control to see a different scene because that npc has a breeding kink, variations on that could also be whether the MC knows this or not also what MC's feeling is on that. Maybe an NPC really likes girls already pregnant because they don't like condoms for instance and knows since they are pregnant he can't get them pregnant.
There could be potential brothels that require no birth control because of xyz reasons, potentially surrogacy stuff as another way to get money.
Imo it is more about ways to get pregnant and the reasoning behind it I am more interested in, as well as the MC feelings on the matter during and after. Too many games lock it behind a gameover/ending which is boring or it is you get instant pregnant and birth and nothing happens after that if you ever get pregnant again those same scenes repeat.
A couple games I have played and think have better pregnancy content are which you can take some inspiration from are.
Girl Life(when I last played years ago that was one of the better parts of the game)
Accidental Woman(Imho, Progenerate job and the stuff around pregnancy are the best parts of that game so far)
Hazumi and Pregnation( Base game is pretty good, but the mods add a good amount of things)
X-change life(Some of the better flavour text when certain things are done but only really for impregnation kinks, the fast forward through pregnancy kind of annoying)
There are many more games but these are the ones I can think of off the top of my head.
