Well, it's not necessarily the visible pregnancy that is the turn on. Maybe it's more an impregnation fetish than a pregnancy fetish. Not everyone who has an impregnation fetish finds pregnant women attractive, but the act of impregnation is the interesting part.
Be the impregnation intentional, or unintentional, or even unknowing. the spectrum probably is pretty broad.
I'm gonna make a rare non-joke or mocking reply:
I can see both sides here as, if the story is ever finished and not abandoned (or real world events don't render all this pointless) it won't be for 20 years so who cares?
Yet, if we pretend it finishes it'll be in the stupid idea of a month in her life. It's funny to remember that each 4-12 month update is just a few hours of her life, lol.
Anyway, stupid pacing aside, after some events I was privy to IRL I can see why people are turned on by pregnancy. It's not a fetish for me but I went from being very turned off by it to understanding what can be hot about it to people (and even myself in some scenarios).
I'd guess it's the idea of marking a woman, a further way to own or use her. "Not only did I fuck her but I fucked her without a condom...and not only did fuck her without a condom but I came inside if her...and not only did I cum inside of her but my genetics mixed with hers to form a child that will grow inside of her".
I can see why that's hot for many as even I went from it being gross to a slight turn-on due to a couple IRL instances for me.
To show Sophia fertilized/impregnated it wouldn't take much and Sophia wouldn't even have to know herself. Technical fertilization takes no time at all in some cases. A man's nut can meet and fertilize a woman's egg before you both go to sleep after fucking. It can happen quickly in comparison to the rest of the events that take place from fucking to the 9 month process before birth.
So to end the story with that info shown or just show after one encounter where some character nuts in her (20 years from now lol) can be done. It doesn't have to be a case of her showing a baby bump or even taking a test.
On the topic in general, I even made a comment about this in the fan art thread recently. It just makes sense to me that if an artist is constantly showing her getting rawdogged and nutted in, make the occasional render showing the common result of that act. I was surprised by how few renders of her pregnant there were just due to wanting fresh ideas and such. I don't necessarily want more but I'm surprised there isn't more.