well it's not so much that it'd be a mountain to implement pregnancy into this game, when you consider everything else either already in the game, or the loose plan for the future. But you're right, pregnancy should be one of, if not THE, last thing(s) that get implemented.
we still have several girls that have been in the game since Day One, Nurse Anne, Social worker Courtney, Doctor Ivy, several schoolgirls, and even those two big name superhero's Billy bumps into on his first day (been a while since I've made a new save, and dont really want to start a new one to get it)
Then we need stuff like the Defense Club quest arc actually REALLY progressing, whether Billy joins the city's Superhero Initiative as a real hero with the existing pair (or even more, but lesser heros), the supervillain arc (some Pinky and the Brain "try and takeover the world" sort of thing).... compared to all that, pregnancy should almost be cake to program (a shit ton of cake, but still cake)