I think this is pretty common fetish in JP. Many games if you search with filter on dlsite for example. Tricky will be to find which ones are limit to small scenes like BAD END scenes and which one have function of pregnancy and birth.
Some RPG maker stuff maybe
i think many of circle gyu rpg games like sword princess cistina or treasure mai.
I have already. Most run in RPG Maker. You can use google to search. You need install RTP for gameplaying. This is free but have several different RTP and which you need depend on game you choose. Check game requirement. Sound difficult but actually very easy.
For Mayuri, this is a flash game. It can run in browser or flash projector from adobe.
All mention game can be got from dlsite. They sell game of erotic nature and many have demo. You can run search and use filter for pregnancy to see what is available. Some game may be only in japanese language but all mentioned games have english patch. You can use google to find.