Making childbirth look realistic is nearly impossible in Koikatsu.
The difference between Rose and Nina is obviously their chara model physique. And then there's the fact that Rose had more than one.
Add to that the instability of Koikatsu Studio. There are errors in the code where - in one instance, you can attach objects to characters and vice versa, and the next load of that save file, the function does not work at all. What we did with Nina worked. We tried it A DOZEN times with Rose but it would not work, even after uninstall, reinstall, upgrade, update, and reinstall again (all of which took 9 full days of downloads and installs)... So it was attempted, but the program failed on us. (Technical info: Route Parent attachment of character only works 1/2 the time, meaning having characters physically move around is impossible 1/2 the time.)
The only major change to any character in this game is when they get corrupted.
Rose is still nerdy Rose. She was simply offered a chance to make a ton of money. She saw it as a solution to the financial distress of her mom and sister after MC vanished on them unexpectedly on Day 12. She was given body modifications as part of the contract. Her makeup is the organization standard.
Other characters change in Chapter 3 because they likewise become corrupted.
It's a bit humorous to us, since so many players keep asking for more clothing options, different makeup, and more hair styles. Changing the appearance is not something we actively chose to do outside of showing corruption or romantic connection to MC.
Regarding multiple sex events for every pregnant character... We have done that for the main love interests, but not necessarily for the rest. There are over 30 girls, at least half of whom may get pregnant before the end of the game. We appreciate the idea, but don't want to get repetitive, as there are only so many (few) positions that work with a baby bump.