Figured I'd shout out a good creator I found on itch. He has a few RPGmaker games with pretty good pregnancy systems. Worth noting is that I think english is his second language, so some of the dialogue can get shaky. Also, most characters can grow dicks and have them while pregnant, but can be toggled if that's not your cup of tea. Finally, incest warning for pretty much all of his games as you can have events with characters you've birthed.
Para Ark: Sci-Fi RPG where you play as copyright-free samus. Get bred by monster enemies to give birth to monsters you use as party members. A little grindy, but like the others can be sped up by editing save files.
https://st-hot-dog-king.itch.io/para-ark
Hell After School 2: 2D action platformer. I only played a little bit of this one before I got bored, mostly because it controlled pretty bad.
https://st-hot-dog-king.itch.io/hell-after-school-2
Worm Nest Plague (WIP): His latest game, not finished yet. Kind of like Bowser's inside story, where you play as an anthropomorphized parasite infecting a girl's body. I'd keep an eye on this one, it's shaping up to be his best yet.
https://st-hot-dog-king.itch.io/worm-nest-plague