That's why if the fetish involves changing your body in some extreme way it's best to keep it for ficticional characters and not real life people.
I mean there's fat fetish sex workers (I can't imagine there'd be any of pregnancy due to the whole child thing), and they either have to eventually stop at a certain weight or lose most of it, else they end up as bedridden blobs with horrible health that die from a heat attack in their 30s. And even then, if they stop too far in they'll continue to suffer from stuff like knee issues and diabetes for the rest of their lives.
A lot of people are into the super exaggerated versions of both fetishes (near blob/blob for the fat ones and some octomom shit for the pregnant ones), and though sex workers try, it's not something that's obtainable IRL and shouldn't be attempted, imo.
If you truly want someone IRL to fawn over fetish-wise then settle for something much tamer like having your partner be chubby/a little bit fat or take a lot of pictures of them during their pregnancy.