itch.io

If you have pregnancy enabled, late in the game (after finishing the chapter 2 main story, basically) you can tell each maid whether to take contraception or not.

If they don't take it, there's a random chance to impregnate them every time you do it inside, but the chance increases every time it fails, so eventually it'll happen.

Then there'll be a notice in their character info screen saying they're "probably" pregnant, which unlocks 2 pregnancy scenes (1 SFW and 1 NSFW) for them on the carnal crystal.

Hope that helps.