I was thinking the pregnant version would overwrite the normal version after you have an encounter where you impregnate them, and then the next time you encounter them they'd be back to normal.
It'd also be good to have the encounter where they are back to normal mention how they gave birth offscreen. Although, an egg laying scene for a pregnant scalie would be pretty amazing too. (Yes I am shamelessly biased towards scalies lol)
Doing it this way is about the simplest and easiest way to do it that would still feel quite satisfying imo
That being said, I'm sure there's the potential for a much more in depth system here if Finchly thinks it's worth the effort.