I going to be honest about this idea. The idea about getting pregnant and giving birth in this game, in a programming way, it's kinda impossible (or maybe not, we will see). Well, you may think it's kinda interesting for gameplay perspective, but for immersion, it's kinda weird to have either the Chad muscular alien or big ass praetorian xenomorph pop out of her body in a full grown size, yea... this might be game breaking thing.
OK , so what about this idea, after you get impregnated, you have a time limit to reach a nearest save block before a time limit run out. If you did not make it to the save block in time, it's game over.
I mean I'm not against your idea or anything, BUT if MC gave birth to a full grown ass alien, it's not immersive. If you want her to give birth in a immersive way, the object, sprite, animation of baby alien is needed to be created. Who knows, maybe someone will find a way to make this work!