I believe that everything related to the fetish of pregnant people is the part that happens before (how the person gets pregnant / impregnation).
Does the woman want to get pregnant ? Is it a so-called "risky" creampie (without warning + the woman isn't taking any contraception + is on a risky day) ? Some of the dialogue during sex scenes can be important.
Is it a forced pregnancy :
- after a rape ?
- after long attempts of discreet insemination during sleep (mainly linked to incest) ?
- after being captured by goblins or another fanciful race ?
- after falling into a trap with a tentacle monster ?
- etc...?
The pregnancy part is often linked to the end of the game, but it also depends on the context of the game you want to make.
There's also the whole lactation part, with milking machines for example, which could be exploited (quite rare in the AVNs I've played).
Personally, I'd like a game that focuses on the pregnancy part itself, and especially the long 'evolution' of the woman's body during the different months/weeks, but I'm not sure this is a subject that interests many people because the body becomes deformed and is no longer 'perfect' as it's usually portrayed in AVNs.